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Topics - johnd

Saturday 24 June 2017

Strumble Head [SM896414]

We assembled at the free car park near to the lighthouse on the headland. The weather was relatively calm with light cloud. We walked  to the WW2 look out building which is now used by bird watchers and tourists, then make our way down the grassy slope to the rocky foreshore. Here we were able to examine the type locality of the Strumble Head Volcanic Formation of Middle Ordovician age (Llanvirn Stage). The volcanic formation is over 1000 metres thick on Strumble Head and formed of basaltic pillow lavas. Each pillow is enclosed in a thin layer of glassy lava representing a chilled margin. Internally the pillows show concentric zones of vesicles, some of which are filled with calcite or dark green chlorite. The vesicles were originally gas bubbles in the molten lava. The spaces between adjacent pillows are sometimes filled with hydroclastites that are aggregates of volcanic glass produced by rapid quenching of molten basalt as the lava flow came into contact with sea water. It is thought that the lava was extruded from submarine fissures and was rapidly cooled under water producing the glassy skin which prevented gas from escaping. Eventually pressure built up within the pillows as new lava was extruded and the outer skins ruptured producing new pillows.
From the lookout building we followed the coast path eastwards to Pen Caer that is formed of a large dolerite intrusion where the cliffs show strong vertical jointing. On the east side of the headland the headland you can look over Pwll Bach [SM 902413] to the Carreg Gybi promontory where there is another dolerite intrusion. A well bedded sequence of light coloured flinty mudstones, felspathic sands and ash bands containing spilite pebbles dip steeply seawards into the bay. These beds are considered to be formed of the disintegration products of contiguous lava flows that have been subjected to deep weathering and more recently differential erosion has resulted in a strong ribbing effect.

We drove along the country lanes to Mathry where refreshment facilities were available in the Farmer's Arms and the local teashop. From Mathry a minor road leads directly to Abercastell where a small car park overlooks the inlet.

Abercastell [SM852336].
We Followed the coast path on the west side of the estuary. In several places the track is cut through outcrops of the Penmaen Dewi shales that display strong cleavage dipping about 700NE. After about 500 metres the path starts to climb steeply over a massive dolerite intrusion that is continued on the opposite side of the inlet on Ynys y Castell. The north side of this island displays the dolerite  cross cutting  bedded rhyolitic agglomerate. Next we followed the path across the fields  towards Ynys Deullyn where the Aber Mawr Shale Formation is well exposed in the steep sided faulted cleft that separates the island from the mainland. About 300 metres along the coast to the west there is a spectacular natural arch where the cleavage planes are dipping steeply NW but the bedding planes can clearly be seen above the arch dipping at about 300SE.
Geology / Geology Group Diary (21)
June 19, 2017, 10:04:05 pm
The Geology Group met at Merlin's Bridge Community Centre at 10.30 am on Wednesday 14 June 2017. The topic considered was:


Using radiometric dating methods the oldest rocks in Pembrokeshire have been dated approximately 643 Ma in the late Precambrian. These occur as a faulted slice of an igneous intrusion (diorite) that extends E-W through Johnston to the southern side of St. Bride’s Bay. The diorite is well exposed in Bolton Hill Quarry near Tiers Cross where it provides an excellent roadstone. In the St. David’s peninsula, the volcanic sequence ( lavas and tuffs)  is somewhat younger, around 610-575 Ma and this is intruded by the St. David’s granophyre which forms the rugged coast around Carreg Fran. Roch Castle, Plumstead Rocks, Poll Carn and Maiden Castle are outcrops of brecciated rhyolite lava that stand as isolated tors representing the remnants of explosive volcanic outbursts during  late Precambrian times.

During Cambrian times [543-490 Ma] marine conditions existed over much of Wales. Turbidite mudstones were deposited on the continental slopes of the subsiding Welsh Basin, as seen in the Harlech Dome, whilst in North Pembrokeshire, in the shallow shelf seas on the margins of the basin, conglomerates and flaggy sandstones were laid down such as the Lingula Flags at Solva and the purple Caerbwdy Sandstone used in the building of St David’s Cathedral. Also mudstones rich in the fossilised remains of trilobites and brachiopods give an indication of the abundance of marine invertebrate life that developed during the Cambrian. The well-known trilobite Paradoxides davidis has been found in the Middle Cambrian shales at Porth y Rhaw near Nine Wells. The best specimens of this large trilobite are displayed in the National Museum of Wales.

By the Ordovician period [490-443 Ma] igneous activity was widespread across Wales as shown by the volcanic rocks of Rhobell Fawr and Cadair Idris in North Wales and the spectacular coastal exposures of the Strumble Head pillow lavas in north Pembrokeshire. These lavas were erupted on to the sea floor as shown by their outer skins that formed by quenching under water. The ‘spotted’ dolerites of Mynydd Preseli as seen on Carn Menyn, were probably the source of some of the Stonehenge bluestones, and the massive layered gabbros of St. David’s Head are examples of igneous intrusions. Many early Ordovician volcanoes were associated with island arc development on the edge of the Welsh Basin and vast quantities of pyroclastic ash and volcanic debris were deposited in the surrounding seas. At the same time, marine sedimentation was continuing to lay down thick muds that became sandwiched between ash layers on the sea bed. The mudstones contain the remains of marine organisms including the well-known ‘tuning fork’ graptolite Didymograptus murchisoni that is found in the Caerhys shales at Abereiddi.

In central Wales – around Aberystwyth, for example – there are great thicknesses of turbidite sandstones and graptolitic shales that were deposited within the deep waters of the Welsh Basin during the early part of the Silurian [443-418 Ma]. However, Pembrokeshire was located on the southern margins of the basin, and here the Skomer Volcanic Group represents the products of volcanism that occurred in shallow shelf seas. Some of the best exposures of lava flows can be seen at Martin’s Haven, Marloes Sands and Skomer Island. The lavas are often interbedded with sandstones and calcareous mudstones that contain a variety of brachiopods and corals. This classic Silurian sequence is well-exposed at Marloes Sands where in the 1960s pioneering research was carried out on brachiopod communities to demonstrate how they could be used as marine depth indicators. Lingula communities generally occupy shallow littoral environments; Eocoelia communities are found in water about 20 metres deep and Costistricklandia communities inhabit depths of up to 50 metres off shore. Thus the changes in brachiopod species within the strata outcropping between the Three Chimneys and about the first 20 metres of the Coralliferous Group suggest marine transgression with deepening water conditions.
Geology / Geology Group Diary (18)
March 08, 2017, 10:51:43 pm
The Geology group met at 10.30 am at Merlin's Bridge Community Centre (Village Hall) on Wednesday 8 March 2017. The monthly topic was:
The River Wye meanders down through Ross and Monmouth to join the Severn south of Chepstow. One of the most scenic areas of the river is around Symonds Yat about 8 kms NE of Monmouth. Here the river swings round in an exaggerated loop crossing the junction between the Old Red Sandstone and the Carboniferous Limestone several times and paying no attention to the geological structure. During Pleistocene times when sea level was 200 metres higher than today the River Wye meandered over a relatively flat surface but as sea level began to fall it started to cut down to a lower base level forming a gorge with incised meanders cut into the limestone. Symonds Yat is a limestone hill forming the core of a meander loop. Yat Rock is a good example of the Lower Dolomite that outcrops near to the base of the Carboniferous Limestone sequence. Dolomite is a type of limestone that is formed of calcium and magnesium carbonate (CaMg) CO3. One of the most distinctive rocks in this area is the Quartz Conglomerate. This marks the unconformable base of the Upper Devonian strata and since it is a resistant rock it forms a strong scarp feature particularly around Huntsham Hill and Coppet Hill. Due to the numerous joints in the conglomerate many huge blocks have broken off and fallen down to the River Wye.
Another interesting feature of the River Wye can be seen near to the village of Newland, about 3 kms SE of Monmouth where there is a large abandoned meander. However, the floor of this fossil meander is about 125 metres above the level of the River Wye so the meander neck must have been cut off probably during the late Pleistocene and the main river then continued to incise its valley down to its present level.
The Tintern Sandstone overlies the Quartz Conglomerate and it forms the main building stone in Tintern Abbey that stands alongside the River Wye.. However, the Lower Wye re enters the Carboniferous Limestone a few kms south of the abbey and there are some spectacular limestone cliffs at Wintour's Leap, near Chepstow.
The Forest of Dean occupies a syncline of Carboniferous rocks between the rivers Wye and Severn. The syncline is asymmetrical with the strata dipping more steeply on the east side than on the west; and the basin is elongated along a N-S axis. This tectonic feature was formed by the Variscan earth movements at the end of Carboniferous times. The Forest of Dean Coalfield is surrounded by Carboniferous Limestone (354-327 Ma) the outcrop of which is wide in the west and narrow in the east where the dip is steep. In the disused  Shakemantle Quarry near Ruspidge the well jointed Lower Dolomite is dipping at about 800W where the outcrop is only 500 metres wide.
One of the most important minerals found in the limestone is iron ore which occurs as haematite Fe2O3 within solution fissures in the limestone. The iron minerals must have accumulated in the oxidising environment of the Permian deserts, then after torrential rains iron rich waters would have descended into the underlying limestones and entered the joints, widened them by solution action and finally precipitated the haematite. Iron ore was mined throughout the Middle Ages particularly in shallow workings in the west but later deep shafts were sunk into the steeply inclined limestone in the east. The local name for iron workings is 'scowles 'and today many of these are hidden away in the woodlands.  The ore was smelted in small furnaces using charcoal from local trees but towards the end of the 18thC supplies of timber were running out and the iron manufacturers turned to coke made from local coal. There are still hammer ponds in existence which provided water power to operate the bellows and hammers in the forges. Cannop Ponds were formerly a site of iron smelting.
A strong unconformity separates the top of the Carboniferous Limestone (which is actually the Drybrook Sandstone) from the Upper Coal Measures (325-300 Ma) that form the Forest of Dean Coalfield. The Millstone Grit and the Lower and Middle Coal Measures are missing so that the basal conglomerate of the Trenchard Group lies directly above the unconformity. Approximately 99% of the strata in the coalfield is sandstone and shale but  there are several thin seams worked by drift mining. The Hopewell Mine that worked the Yorkley Seam is now a Mining Museum. However, the main commercial seam is the Coleford High Delf which has an average thickness of 1.3 metres. This seam lies at the base of the Pennant Sandstone Group which contains some massive red sandstones much used for building stone. Pennant Sandstone from Bixstead quarries near Coleford has been used in many public buildings including Shire Hall, Gloucester and the University of Wales, Aberystwyth. Blocks of sandstone are extracted from underground where the quality of the stone is superior to that near the surface. Many of the quarry faces show cross stratification indicating deposition under deltaic conditions. The casts of large tree trunks such as Sigillaria occur in some of the beds; evidence of extensive forest growth on the deltas.
Another major unconformity separates the  Upper Coal Measures from the Mercia Mudstones ( Keuper Marl) of the Triassic Period (248-205 Ma)  that are exposed on the west bank of the Severn at Sedbury Cliff near Chepstow. These marls were laid down in mudflats and lacustrine areas of inland drainage in the tropics and they contain few fossils although reptilian vertebra are found. However, the foreshore at Sedbury is strewn with fallen blocks from the thin overlying layers of Rhaetic shales and Lower Lias (Jurassic) limestones. The beach is a well known for fossil collecting as the Lias blocks yield numerous ammonites and the bivalve Qstrea liassica. 

Geology / Geology Group diary (17)
February 08, 2017, 08:29:09 pm
The Geology Group met at Merlin's Bridge Village Hall at 10.30am on Wednesday 8 February 2017. There was an excellent turn out of 18 members. This month's topic was:
Edinburgh has long been seen as the cradle of Geology which developed as a scientific discipline during the Scottish Enlightenment in the late 18th century. James Hutton (1726-1797) was a physician and natural philosopher who spent much of his life studying the rocks around Edinburgh and formulating theories as to their origin.  Bishop Ussher (1581-1656) using biblical data had calculated that the earth formed in 4004 BC, but Hutton was convinced that the earth was millions of years old with "no vestige of a beginning and no prospect of an end". We now know that the earth is some 4.5 billion years old. Hutton argued that the uplift, erosion and deposition took enormous lengths of time and that present day physical processes were the key to understanding the past. In his book Theory of the Earth Hutton put forward this idea of uniformitarianism; that the earth's surface had been changed immeasurably slowly as opposed to catastrophism or violent upheaval. He also considered that the earth's interior was hot and that molten rock was erupted at the surface which he explained with reference to Salisbury Crags in Edinburgh. This idea brought him into conflict with the current Neptunist view that all rocks had been precipitated out of the Great Flood.
Charles Lyell (1797-1875) was a Scottish geologist who popularised Hutton's ideas in his 3 volume Principles of Geology which in turn inspired Darwin on his voyage in the Beagle. The frontispiece of Lyell's book showed a drawing of the Temple of Serapis in Pozzuoli in Southern Italy. Three columns of this Roman ruin still stand and a line of mollusc borings can be seen about a metre above the base of the columns. The evidence suggests that the building had slowly sunk beneath the sea and later been uplifted which Lyell used as support for the concept of uniformitarianism.  Lyell also produced numerous scientifical papers on volcanoes, earthquakes and stratigraphy and enthusiastically advocated Hutton's dictum that 'the present is the key to the past'.
Igneous rocks in Edinburgh. During the early Carboniferous Period (354-323 Ma) considerable volcanic activity occurred in the Central Lowlands of Scotland and in Edinburgh where Arthur's Seat Volcano erupted explosively. Today we can see the agglomerate (broken pieces of lava and ash) in Queen's Drive (Holyrood Park) which blocked the volcanic vent. Some of the basalt lavas that formed the volcanic cone can be seen on Whinny Hill to the east of the city. Castle Rock on which Edinburgh Castle stands, is the site of a parasitic vent. Salisbury Crags are formed of a massive dolerite sill intruded into the west side of the volcano. At Hutton's Section (SSSI) you can see the contact between the dolerite and the underlying Carboniferous sandstone which has been ripped up and baked by the intrusion. Bass Rock in the Firth of Forth is an eroded volcanic plug marking the site of another Carboniferous vent.
Hutton's Unconformity, Siccar Point In 1788 Hutton visited Siccar Point on the Berwickshire coast, 40 miles from Edinburgh near to the village of Cockburnspath (on the A1 south of Dunbar). Here he discovered two totally different types of sedimentary rock in contact with each other. The underlying Silurian greywacke was steeply dipping and its surface was eroded. Resting on this surface was a horizontal basal conglomerate of Old Red Sandstone. Hutton inferred from the sharp junction between the strata that an enormous time interval was required for the underlying rocks to have been uplifted, folded and eroded before the overlying conglomerates and sandstones were laid down. The fundamental geological principle of 'deep time' was thus established and it would later be shown that a time gap of some 65 million years was represented by the Siccar Point unconformity.
Oil Shale in West Lothian Oil production began in Scotland in 1851 when James Young opened a refinery near Bathgate where he processed local cannel coal by heating it in a large retort to extract oil which could then be further refined into paraffin, lubricating oil and motor spirit (petrol). At the time whale oil was used for oil lamps but this was soon replaced by cleaner paraffin lamps. Factories also needed oil to lubricate machines and by the turn of the century motor spirit was being used to fuel the internal combustion engine. Supplies of local coal were soon used up and so Young then turned to the oil rich shales that were plentiful in West Lothian coalfield. Today the legacy of the oil shale industry can be seen in the huge tips or'bings' of burnt shale that litter the landscape. About 8 tonnes of shale were required to produce 10 barrels of oil. The slow decline of the industry began in 1924 when a refinery was opened at Grangemouth on the Firth of Forth that was designed to process crude oil imported from the Persian Gulf. Refined oil could then be produced at a fraction of the cost of Scottish shale oil.
What is Oil Shale? Cannel coal or candle coal is a hard bituminous coal that burns with a bright flame and was used for domestic fires in the 19th C. It was formed in coal swamps on Carboniferous deltas from plant remains (tree ferns, horsetails, club mosses). Oil shale is also highly bituminous but is formed in a lagoonal environment at an earlier stage in the cycle of sedimentation or cyclothem during Upper Carboniferous times (311-290 Ma). However, repeated marine submergence and terrestrial emergence ensured that repeated cyclothems containing coal seams and oil shale bands were formed.
Forth Bridge. The famous cantilever railway bridge was opened in 1890. The site at Queensferry was where the Firth of Forth narrows and where there were firm foundations for the bridge piers. A massive dolerite intrusion extends from North Queensferry to Inchgarvie and West Lothian oil shales outcrop on the south side. In 1964 a suspension bridge was opened for road traffic and a new Queensferry bridge is due to open in 2017.
Geology / Geology Group Diary (16)
January 11, 2017, 09:01:16 pm
The Geology Group met at Merlin's Bridge Village Hall on Wednesday 11 January 2017 at 10.30 am. The topic was 'The GEOLOGY OF THE MENDIPS AND SURROUNDING AREA'.
The Mendip Hills lie to the south of Bristol and extend WNW-ESE from Weston super Mare to Frome in Somerset. This axial trend reflects the influence of the Variscan earth movements that produced a major asymmetrical anticlinal structure plunging eastwards. Since the main thrust came from the south as the Rheic Ocean closed, the dips on the north side of the Mendips are much steeper than those on the south. Since the oldest rocks lie in the core of an anticline we find the Upper Old Red Sandstone in the centre of the Mendips where it forms the upland known as Black Down. The Carboniferous Limestone has been eroded over the centre of the anticline and now outcrops on the flanks of the Mendips. The northern outcrop is well seen in Burrington Combe which is one of several valleys cut into the limestone. However, during Triassic times flash floods would have dumped vast quantities of limestone debris and sand into these valleys. Today erosional remnants of the Dolomitic Conglomerate (which in fact is a breccia) can be seen resting unconformably on the sides of Burrington Combe. Thus much of the Mendips have been exhumed from beneath a Triassic cover.  The steeply dipping limestone beds are well seen at the Rock of Ages which is where the Rev.Montague Augustus Toplady is said to have sheltered in a cleft during a storm in 1763. According to the apocryphal story he received divine inspiration after which he wrote the classic geological hymn 'Rock of Ages'! Aveline's Hole is a fine example of a swallow hole that follows the inclined bedding plane and leads down to two large chambers where the remains of a Mesolithic cemetery have been found.
Cheddar Gorge on the south side of the Mendips was cut by meltwater during cold periglacial phases of the Pleistocene Ice Age when permafrost would render the limestone impermeable. In warmer interglacial times the waters would descend into underground passages and caves leaving the gorge dry, as it is today. The caves contain the remains of animals that lived in Devensian times and the bones of 'Cheddar man' in Gough's Cave have been dated to around 7150 BC.
Ebbor Gorge lies about 8 kms to the south west of Cheddar where the Ebbor Thrust has forced the Carboniferous Limestone over Namurian quartzite that is exposed by a stream near the entrance to the gorge. The thrust zone extends along the SW margin of the Mendips and demonstrates the powerful effect of the Variscan earth movements. Wookey Hole near Wells is a magnificent swallow hole leading to some 25 underground caverns through which flows the River Axe. Human and animal remains indicate that the caves have been used since Palaeolithic times. Wookey Hole is a SSSI but like Cheddar, it is highly commercialised and a tourist' honey pot'.
The River Mells flows through Vallis Vale about 1.5 kms west of Frome. Here the steeply bedded Carboniferous Limestone is overlain by horizontal beds of Inferior Oolite (Middle Jurassic). This is a classic unconformity that was first described by Henry De la Beche (1796-1855) the first director of the Geological Survey. Here the limestone erosion surface shows worm borings and bivalves and immediately above is the basal conglomerate of the Inferior Oolite marking a time gap of some 150 Ma.
The Avon Gorge which is spanned by the Clifton Suspension Bridge, cuts through a complete sequence of Lower Carboniferous rocks. In 1905 Vaughan pioneered the use of an assemblage of brachiopods and corals to create fossil zones K,Z,C,S & D to subdivide the strata [Cleistopora, Zaphrentis, Caninia, Seminula,& Dibunophyllum]. Today these zones have been superceeded by a classification based on 6 cycles of marine sedimentation during the Lower Carboniferous.
Portishead Point is formed of Carboniferous Limestone overlain unconformably by the Dolomitic conglomerate, whilst the coastal section along Kilkenny Bay shows the Dolomitic conglomerate resting directly on the Old Red Sandstone. The structural trend of the Mendips  is clearly demonstrated in the Weston super mare area where the three headlands of Brean Down, Worlebury Hill and Middle Hope are all formed of Carboniferous Limestone. At Swallow Cliff on the north side of the Middle Hope headland the limestone contains a volcanic horizon formed of a basal tuff overlain by some 10 metres of basaltic pillow lavas. Thus it appears that during Lower Carboniferous times whilst the limestones were being laid down in shallow shelf seas fringed with coral reefs, some volcanoes were erupting submarine basalt lavas.
Aust Cliff is where the first Severn Bridge was built in the 1960s. The cliff itself is formed of Red Triassic marls at the base overlain by Rhaetic shales and thin bedded Liassic limestones, but these relatively soft rocks rest on a foundation of Carboniferous limestone. This extends out beneath the River Severn and the first pier of the bridge is built on the solid Ulverstone Rock. Unfortunately Carboniferous mudstones covered by Triassic sandstones outcrop on the Beachley side of the river and so the second pier had to be sunk 15 metres down into the mudstones to secure a firm foundation.
Geology / Geology Group Diary (15)
December 14, 2016, 09:37:44 pm
The Geology Group meeting took place at Merlin's Bridge Village Hall at 10.30am on Wednesday 14 December 2016. This month's topic was:
These two counties in North East England extend from the River Tees in the south to the River Tweed on the Scottish border. The Tees, Wear and Tyne drain the western flanks of the North Pennines and the R.Coquet flows off the Cheviots in Northumberland.
Some of the oldest rocks in the region are the igneous rocks of the Cheviots that are of Lower Devonian age (417-391 Ma). The Cheviots are formed of andesitic lavas, agglomerates and ashes that represent the eroded remnants of a massive Devonian volcano. These volcanic rocks were intruded by a granite pluton (c.380 Ma) which occupies the central summit of the Cheviot hills. The most northerly stage of the Pennine Way crosses the Cheviots before descending to Kirk Yetholm.
Much of NE England is formed of rocks of Carboniferous age. During Lower Carboniferous times (363-325 Ma) over 1600 metres of Yoredale strata were laid down in the Northumberland basin. (By contrast only 300 metres of limestone was deposited on the Alston Block). The Yoredale sequence contains numerous cyclothems or cycles of sedimentation. Each one starts with a limestone formed under shallow marine conditions, this is followed by siltstones which then passes into cross stratified sandstones laid down in a deltaic environment. Finally, the swampy forested delta top will give rise to the development of thin coal seams. The Scremerston Coal Group within the Yoredale sequence in Northumberland has one 2 metre thick coal seam that has been worked extensively. Thus during the Lower Carboniferous whilst limestones were being deposited over the Pennine blocks, large deltas were spreading across the Northumberland basin to deposit shales, sandstones and coal seams. However, in the Alston Block (North Pennines) the Great Limestone contains a coral band known as the Frosterley Marble. This is a coral limestone that can be polished to look like marble and it is used in the nave of Durham Cathedral as decorative supporting columns.
By Namurian times (325-315 Ma) the deltas had become more widespread and extended farther south as increasing quantities of sediment were brought down by rivers from the northern land mass. Thick coarse grained cross bedded sandstones known as Millstone Grit were deposited in cyclothems that contained thin coal seams. By Westphalian times (315-300 Ma) cyclic deltaic sediments included beds of shale and sandstone topped by thick coal seams. The Northumberland Durham coalfield supplied 'sea coal' to London in Shakespeare's day; the coal was exported from Tyneside and other coastal ports. By the 19th century the iron and steel, engineering and shipbuilding industries were powered by coal that was mined in the exposed coalfield which extended eastwards beneath the North Sea and formed the concealed coalfield that lay beneath younger Permian strata.
In late Carboniferous times sheets of molten magma were injected into the rocks of North East England. One of the most famous of these intrusions is the Whin Sill, a 30 metre thick dolerite sill that covers an area of over 500 km2 yet it is exposed at the surface only as a narrow outcrop from the Farne Islands, through Northumbria to Cross Fell on the Pennine escarpment and in Teesdale (High Force waterfall). Hadrian's Wall follows part of the outcrop of the sill which provides an excellent defensive feature.
The Permian rocks (290-245 Ma) in south east Durham are represented by dune bedded sandstones reflecting the desert conditions that existed at the time. These cross stratified sandstones rest unconformably on the Coal Measures. However, the Zechstein Sea gradually transgressed across the arid Permian landscape first depositing the Marl Slate, which is a bituminous shale containing fish remains. Next  the Magnesian limestone was laid down and this now forms a well defined escarpment stretching from Sunderland to Teesdale. This limestone is formed of the mineral dolomite or calcium magnesium carbonate. It contains many marine fossils such as brachiopods, gastropods and bivalves. A concretionary form of the Magnesian limestone known as the 'cannonball limestone' is well developed in the Roker area of Sunderland. As evaporation took place within the Zechstein Sea, a sequence of repeated cycles of dessication occurred in which salts were precipitated in order of increasing solubility. Starting with the least soluble calcium carbonate CaCO3,then gypsum CaSO4, halite NaCl, magnesium and potassium salts (most soluble). The fluctuating level of the Zechstein Sea would account for the repeated cycles of dessication.
Geology / Geology Group Diary (14)
November 09, 2016, 08:26:07 pm
The Geology Group met at Merlin's Bridge Village Hall at 10.30am on 9 November 2016. A total of 18 members attended. The topic was:
The Thames Basin is a shallow syncline consisting of Palaeogene (Lower Tertiary, 65-24 Ma) sediments resting on Cretaceous rocks. The basin is bordered by chalk scarps; the chalk of the Chilterns dips south under the Thames valley, whilst the chalk of the North Downs dips north. Most of London stands on London Clay which forms an impervious cap above the Chalk aquifer. This in turn is sealed below by an impervious layer of Gault Clay. Thus rain falling on the chalk hills will slowly percolate through the strata creating a subterranean reservoir within the chalk below London. Artesian boreholes sank in the 19th C were under sufficient hydraulic pressure to supply water to the fountains in Trafalgar Square without the need for pumping. The reason why London has the most extensive underground rail system in the world is because it was relatively easy to excavate tunnels in the soft clay using pick and shovel methods in Victorian times.
The Chalk is Upper Cretaceous (99-71 Ma) in age. It is is a fine grained white limestone formed from calcareous mud containing microscopic coccoliths derived from marine plankton. The Cenomanian transgression in late Cretaceous times covered much of southern England and since there was little sand and mud brought down by rivers, the sea remained relatively clear and the chalk sediment was free from impurities. Fossils are fairly common in the Lower Chalk including ammonites, belemnites, echinoids, bivalves and brachiopods. Micraster, the heart urchin is common as are brachiopods such as terebratulids and rhynchonellids. The Upper Chalk is characterised by the presence of flint nodules which may have been precipitated from silica rich ground waters percolating through the chalk. However, recent research suggests that the flint was formed by the sub surface breakdown of siliceous organisms such as sponges during the deposition of the chalk.
The chalk escarpments and dip slopes of both the North Downs and the Chilterns provide examples of typical chalk scenery. Spring lines occur at the base of the chalk scarps where the underlying Gault Clay causes the water to emerge through springs eg. Silent Pool, Shere, Surrey. Dry valleys are also common where the water table has been lowered since the valleys were first eroded. Clay with flints form a cover on much of the chalk surface. It is thought that it is derived from the weathering of the overlying Reading beds.  Localities where you can study chalk features include Box Hill, nr Dorking; Newlands Corner-Clandon nr Guildford and Beacon Hill (Rowant Aston Nature Reserve), Bucks.
The sediments within the London basin were deposited during the Palaeocene and Eocene periods (65-33 Ma) when the London and Hampshire basins were subject to alternate marine transgression and regression. Initially the sea advanced from the east over the eroded chalk surface and laid down the Thanet Sands which are best exposed along the North Kent coast at Herne Bay.  Later, as the sea regressed the Reading Beds were deposited by meandering rivers flowing eastwards across mudflats . These continental type beds consist mainly of sands and clays but locally where silica cementation occurs, the sands form sarcen stones. These massive stones are often found on the chalk surface after the unconsolidated Reading beds have been removed by erosion. Sarcens can be seen on the Marlborough Downs on the western margin of the London Basin and they were used to build Stonehenge and other prehistoric monuments. Cemented pebble beds also occur such as the Hertfordshire Puddingstone; a superb example of a natural concrete! The succeeding cycle of sedimentation begins with a marine transgression that produced the London Clay up to 150 metres in thickness.  There are beds of calcareous concretions including septarian nodules at various levels within the London Clay. Numerous bivalves, brachiopods and gastropods provide evidence of the shallow marine environment. Sharks teeth are commonly found as fossils. .However, over 500 plant species have been recorded including mangroves, palms, laurel and magnolia which suggests a tropical climate existed in Eocene times. The plants and seeds were probably brought down by rivers and washed out to sea to be preserved in the silty marine sediments. A return to continental type conditions occurred as the sea shallowed and regressed and the Bagshot Sands were deposited in an estuarine environment. These beds are well developed in the western part of the London Basin giving rise to heathlands much used by the military around Aldershot and Bagshot.
Around 400,000 years BP during the Anglian glaciation the proto-Thames flowed along the Chiltern plateau and the present Vale of St Albans towards the Essex coast. As the ice sheet moved slowly southwards the St Albans channel was blocked and the drainage was diverted south through the Finchley depression. This outlet was later blocked by the ice front and the Thames established its modern route through London towards Southend on Sea. At this time the Thames meandered across a wide flood plain some 50 metres above present sea level, but as sea level dropped (or alternatively uplift of the land occurred) the river cut down to a lower base level leaving the former flood plain as the Boyn Hill Terrace. By 200,000 years BP base level had dropped again causing the Thames to cut a new deeper channel leaving its former flood plain as the Taplow Terrace. Finally renewed downcutting  produced the Floodplain Terrace which stands  above the present flood plain. The river terraces are economically important for sand and gravel excavation, reservoirs, water parks (eg.Lea valley) and market gardening on the brickearths particularly in Victorian times.
Geology / Geology Group diary (13)
October 12, 2016, 08:47:42 pm
We gathered at Merlin's Bridge Village Hall at 10.30am on Wednesday 12 October 2016. The topic this month was:
The geology of East Yorkshire consists mainly of Jurassic and Cretaceous strata. The major structural feature of the region is the Cleveland anticline that runs roughly W-E through the Cleveland Hills to Robin Hood's Bay. This was uplifted in Miocene times (like the Wealden anticline). To the south of the axis the strata youngs and dips gently towards the Vale of Pickering.
Lower Jurassic (205-180 Ma) Liassic sediments are well exposed along the coast between Staithes and Robin Hood's Bay.  They underlie the Cleveland Hills which form the northern part of the region bordered by Teesside to the north and the River Esk to the south. The Cleveland Ironstone Formation (Middle Lias) can be seen to the east of Staithes where it was worked in the 19th C for use in the iron and steel industry on Teesside. There are 4 main seams of bedded ironstone exposed on the coast and these can be traced inland where they thicken and outcrop in the escarpment around Guisborough. The harbour at Port Mulgrave was opened in 1856 to handle the output of the coastal mines. The iron ore occurs as an oolitic mudstone rich in siderite and chamosite and was deposited under shallow marine conditions; evidence includes a prolific shell fauna, ripple marks, trace fossils and cross stratification.
The Upper Lias contains the famous Jet Rock which is a bituminous shale horizon containing discrete  masses of the carbonaceous mineral jet. This appears to have been formed from drifted logs of wood which sank into the anaerobic muds on the sea floor and then became compressed under the weight of overlying sediments. The Whitby jet derives from Jurassic plants similar to the Chilean pine or Monkey Puzzle tree. Jet was worked by the Romans and was also popular in Victorian times particularly for mourning jewellery.
The Alum Shales occur at the top of the Upper Lias. These pyrite (FeS2) rich shales formed the raw material for the alum industry that began in the 17thC and continued up until the 1870s. The alum was used in tanning, dyeing and in medicine. The shales were quarried mostly on the cliff tops where the treated waste could be easily dumped on to the shore below. Calcination required alternate layers of brushwood and shale to be slowly heated over a period of a year or more. During calciation sulphuric acid was produced by the oxidation of the pyrite and it reacted with the aluminium silicates in the shale to form aluminium sulphate, The burnt shale was then steeped in water in tanks to extract the aluminium and iron sulphates from the shale. The resulting liquid was then concentrated by repeated boiling, evaporation and crystallisation of the salts. Whilst the iron salts remained in solution the  alum salts ( hydrated  sulphate of potassium and aluminium) would crystallise out. The exact time to cease heating was determined by floating an egg on the liquid!
The Lias of North Yorkshire represents repeated cycles of marine sedimentation, each cycle containing clays, shales, sandstones and ironstones in upward succession. As the cycle coarsens upwards, the water becomes shallower and oxygenation increases giving rise to a profusion of marine creatures including ammonites, belemnites, bivalves and marine reptiles. Robin Hood's Bay is a wonderful fossil hunting locality. Dactylioceras and Hildoceras are two well known ammonites from the Upper Lias, the latter is named after St Hilda who founded Whitby Abbey and is reputed to have turned the local sea serpents into stone which are now found as ammonites!
Middle Jurassic (180-159 Ma) sandstones and shales form the North York Moors to the south of the River Esk. Here the poor acidic soils support only heather and rough grassland. Contrast this with the richer farmland of the Cotswolds that are formed of oolitic limestones of the same Middle Jurassic age. Why is this?  Jurassic sedimentation varied considerably across England and was largely deltaic in Yorkshire as opposed to the shallow marine conditions that existed in the south of England. This was because the underlying basement exhibited a basin and swell topography where areas of subsidence accumulated thick deposits of Jurassic strata whilst areas of uplift were covered with much thinner deposits. The three main swells are Mendip, Moreton and Market Weighton; note that above the latter the Jurassic cover is almost completely removed. The Middle Jurassic of Yorkshire begins with a distinctive marine unit known as the Dogger Formation which contains conglomerates, sandstones and ironstones and is often characterised by the presence of large concretions or 'doggers'. However, most of the Middle Jurassic is made up of deltaic sands and mudstones in which are preserved abundant ferns, conifers, cycads and horsetails (Equisetites). These plants must have been growing on the delta flats and alongside distributary channels in sub tropical climatic conditions. Roseberry Topping, an isolated hill near Great Ayton, is a good example of an outlier capped by deltaic sandstones in which occurs the earliest known plant bed in the Middle Jurassic. The famous Cleveland Dyke runs close to Roseberry Topping in a direction WNW-ESE for nearly 50 kms across the North York Moors. The dyke is formed of basaltic andesite and contains phenocrysts of plagioclase feldspar. It was intruded in Tertiary times from the igneous complex on the Isle of Mull. The igneous rock makes an excellent roadstone and so the dyke has been quarried in several localities around Great Ayton where it forms a deep cutting.
Upper Jurassic (159-150 Ma) rocks form the high plateau of the Tabular Hills on the southern margin of the North York Moors. Since the strata is dipping south there is a well defined escarpment along the north side of the Tabular Hills which are capped by Corallian oolitic limestones and calcareous sandstones. The scenery here is similar to the Cotswolds with rich brown soils and dry stone walls. The undulating plateau is dissected by streams flowing down the dip slope to join the River Derwent in the Vale of Pickering which is underlain by Kimmeridge Clay. The impressive ruins of the Cistertian abbey at Rievaulx stand in an isolated valley on the Corallian dip slope near Helmsley.
Glacial Drainage. During the late Pleistocene the North York Moors were surrounded by ice  from the North Sea and the Vale of York. The eastern end of the River Esk was blocked by ice and a proglacial lake was formed in the Esk valley. Eventually the water level built up to the point where it overflowed southwards across a col at Newtondale cutting a deep meltwater channel. The water poured into the Vale of Pickering where a proglacial lake had formed due to ice blocking the River Derwent's outlet to the North Sea. Overflow from the lake spilled out through the Kirkham Abbey gorge, thus diverting the river south to the Humber; a classic example of glacial drainage diversion.

Geology / Geology Group Diary (12)
September 14, 2016, 11:05:22 pm
The Geology Group met at Merlin's Bridge Village hall at 10.30am on Wednesday 14 September 2016. Please note that the following notes will be available in hard copy at the next meeting.  The topic for the session was....
The oldest rocks in South Devon are found in the most southern part of the county between Start Point and Bolt Tail. The area is known as the Start Point Complex and is formed of alternating zones of grey mica schists and green hornblende schists. The schists have been produced by regional metamorphism involving high temperatures and pressures. The mica schists show strong foliation and are formed from pre existing sandstones and shales whereas the hornblende schists were originally basaltic lavas and tuffs. The Start Point complex is faulted against the Meadfoot Beds in a line running E-W through Salcombe. This fault zone has a counterpart along the northern margin of the Lizard complex in Cornwall. Both areas appear to have been metamorphosed and thrust from the south against the Devonian land mass during early Variscan earth movements. Therefore although the schists were metamorphosed during the Lower Devonian (417-391 Ma) they are derived from much older rocks. Details of the Start Complex thrust structure are clearly seen at Outer Hope near Bolt Tail where a melange of brecciated boulders lies in the centre of the fault zone separating the schists from the Meadfoot slates to the north.
Another more recent feature of the South Hams (undulating land south of Dartmoor and between the rivers Dart and Plym) is the 140 metre marine erosion platform. This is a plateau surface that was cut during Pliocene times when the sea level was much higher than today.  A raised beach with degraded cliffs stand about 10m metres above present sea level around the coast particularly near Prawle Point. This relates to the end of Pleistocene times as sea level rose as the ice sheets melted. Much of the material below the raised beach is formed of periglacial head; angular rock fragments in sandy clay (solifluxion deposit). At Hallsands just north of Start Point there are the ruins of a small fishing village standing on the raised beach. The village was destroyed in the winter storms of 1917. This was a disaster that should never have happened. It was the result of extensive shingle dredging to provide material for extensions to the naval dockyard in Plymouth which began in 1897. The dredgers removed the protective shingle barrier in Start Bay which had previously absorbed much of the energy of the easterly gales.

The large estuary of the Salcombe river bisects the South Hams; its branching outline is characteristic of a drowned river system known as a ria. This was produced as a result of rising sea level around 10,000 years ago when the Pleistocene ice sheets were melting.

The Meadfoot slates are Lower Devonian in age and they can be seen at Tor Cross where the sea has exposed the intricate fold structure of the original bedding and its relation to axial planar cleavage. Immediately north of Torcross is Slapton Ley, a freshwater lagoon protected from the sea by a shingle spit that has grown southwards across the mouth of the River Gara. Meadfoot Beach in Torquay is the type locality for the Meadfoot beds but Middle Devonian (391-370 Ma) limestones form many of the cliffs around Torquay including Hope's Nose. Here on the south side of the headland there is a superb example of a recumbent fold above a gently inclined thrust plane. A well preserved raised beach with pebbles and shells embedded in sand stands about 10 metres above sea level on Hope's Nose. London Bridge is the name of a natural arch near to Dyer's Quarry; where formerly limestone blocks were quarried and exported by sea. Today the floor of the quarry exposes a rich fauna of both single and colonial corals. Triangle Point marks the southern end of Meadfoot Beach. It is a faulted block of limestone where the exposed inclined bedding plane is covered with corals and stromatoporoids. These are mounds of onion like layers which have been secreted by creatures as a protection.
The nearby Kent's Cavern is an example of a cave system that has been formed by underground streams dissolving the limestone and removing calcium carbonate in solution. Kent's Cavern proved to be a rich depository of animal bones when the cave earths were excavated during the 19th century. The bones of sabre toothed tigers, hyaenas and woolly rhinoceros indicated that these animals were probably hunted by Neolithic people who inhabited the cave from time to time.

The railway line between Teignmouth and Dawlish follows the coast where Brunel inaugurated his atmospheric railway in 1846. The construction of this coastal railway opened up some excellent cliff sections in the dune bedded sandstones of Permian age (290-252 Ma). At Coryton's Cove on the south side of Dawlish honeycombed weathered  breccias (alluvial fan deposits) are overlain by aeolian sandstones  (desert dune formations). Cross stratification can be seen in the cliffs north of the town where each set of dunes has been eroded as the wind direction changed and a new set of dunes deposited above on a new surface. Several sea stacks can be seen along this stretch of the coast including the Parson and Clerk near Dawlish. Dawlish Warren is a sandspit that has been built out from Langstone Rock by longshore drift across the Exe estuary. The sand dunes on the seaward side of the spit give some protection to the salt marsh and nature reserve on the inner side.

Budleigh Salterton is a pleasant seaside town to the east of the Exe estuary. On the pebble beach Millais was inspired to paint 'The Boyhood of Raleigh'. The Elizabethan explorer was born at nearby East Budleigh c 1552. Geologically the cliffs at Budleigh Salterton are desert sandstones of Triassic age (252-241 Ma). The pebble beds were laid down by ephemeral braided streams in a desert basin. Oxidation of iron minerals to ferric salts in arid environment gives red colour to the rocks. Stream channels can be seen in cross section in the cliff face. Cobbles, boulders, and pebbles are set in gravel and silty sand. The pebbles are mainly quartzites but there are also clasts of vein quartz, schorl (quartz + tourmaline), porphyry and sandstone. The top of the formation is marked by a distinctive yellow sandy band and a thin layer of black pebbles. These mark an aeolian abrasion surface on which lie polished wind faceted pebbles known as dreikanters. The overlying Otter Sandstones are formed of wind blown desert sands. They contain rhizoconcretions that are carbonate cemented concretions around the roots of primitive Triassic plants.
NB. (i) The Devonian rocks of South Devon are all marine whilst in the rest of Britain Old Red Sandstone was being deposited under arid continental conditions.(ii) The Permian and Triassic rocks make up the New Red Sandstone that was deposited under desert conditions.
Geology / Geology Group Diary (11)
July 14, 2016, 10:56:39 pm
These are the notes for the session on 13 July 2016 at Merlin's Bridge. The topic was GEOLOGY & SCENERY IN NORTH WALES

The oldest rocks in North West Wales are found in Anglesey and the Lleyn peninsula. These rocks of late Precambrian age are known as the Monian Group (at least 615 Ma) and they are well displayed at South Stack and Rhoscolyn on Holy Island off the west coast of Anglesey. The Monian Group contains turbidite sandstones, volcanic ash and pillow lavas and a remarkable unit called the Gwna melange that is made up of a chaotic mixture of blocks and boulders some of which are several hundred metres across. This represents an enormous submarine slide down the continental slope known as an olistostrome. Pillow lavas are well exposed at Newborough on the south east side of Anglesey along the Menai Strait. Lava flows of this type represent submarine extrusions of basaltic lava that formed large globules or 'pillows' as they react with sea water. Much of the Monian sequence has been strongly metamorphosed into schist and gneiss and intensely folded during orogenesis at the end of Precambrian times when a subduction zone existed in NW Wales.

Rocks of Cambrian age ( 544-510 Ma) in North Wales are represented by a thick sequence of marine sandstones and shales (most of which are now metamorphosed into slates). The Llanberis Slate outcrops on the NW side of Snowdon where the formation is over 1000 metres thick. The slates were formed as a result of pressure exerted on the Cambrian mudstones during the Caledonian earth building movements at the end of Silurian times. The platy minerals like mica and chlorite in the mudstones were re orientated by pressure that created cleavage planes often perpendicular to the original bedding planes. The slates cleave into thin layers that make excellent roofing material. In the late 19th C at the peak of production, the slate quarrying industry exported Welsh slate across the world. A more modern use of the disused Dinorwic quarry is the pumped storage hydro electric scheme that was opened in 1984. The turbines are hidden in the old slate caverns and they are powered by water released from a high level reservoir which is refilled at night by pumping water up from the lower lake using cheap off peak electricity.
The Harlech Dome is essentially an inlier of Cambrian rocks uplifted as a result of the Caledonian orogeny. The oldest rocks in the centre of the eroded dome are surrounded by a rim of the Rhinog Grits which are topographically higher (Rhinog Fawr 720m). One of the best walks in the Rhinog National Nature Reserve starts at Roman Steps near Cwm Bychan. Manganese and iron pyrites occur in the shales above the Rhinog Grits and gold was mined in the Lower Cambrian rocks between Barmouth and Dolgellau during the 19th C. Note that the oldest rocks are in the centre of the dome and the youngest rocks that are stratigraphically above the older rocks outcrop on the periphery of the dome.

The Snowdon mountain range is composed largely of Ordovician mudstones and volcanic rocks. In early Ordovician  times (Tremadoc age 510-493 Ma) marine conditions  existed and great thicknesses of muds were deposited in a geosynclinal basin. These muds were later converted by regional metamorphism into the Tremadoc slates. It is interesting to note that many of the trilobites found in the Tremadoc slates have been distorted by pressure during metamorphism. During most of the Ordovician an island arc was established above a subduction zone on the south side of the Iapetus Ocean and numerous volcanoes spewed volcanic ash and lava into the surrounding seas. Vulcanism continued throughout the Arenig and Llanvirn times (493-464 Ma). The most intensive period of volcanic activity produced some highly explosive eruptions which ejected masses of felsic pyroclastic materials and resulted in the creation of a huge caldera of subsidence in Central Snowdonia. The pyroclastics are of two main types: Airfall materials represent the products of nuée ardente eruptions where a glowing volcanic cloud deposited fine ash into the surrounding sea where it was reworked as a sediment known as air fall tuff. By contrast, pyroclastic ash flow materials roll down the volcanic slopes as incandescent avalanches of gas, steam and lava. They became welded together to form ash flow tuffs or ignimbrites. These often contain glass shards and flattened pumice fragments known as fiamme.

One of the best localities to view some of the volcanic rocks is at Cwm Idwal on the north side of the Glyder Range. This is a National Nature Reserve with a well marked path around Llyn Idwal. Near to the start of the trail there are exposures of the Pitts Head Tuff which lies at the base of the Snowdon Volcanic Group. This contains numerous spherical siliceous nodules 2-3cm in diameter. These represent infilled gas bubbles within a pyroclastic ash flow. At the head of the cwm the Idwal Slabs represent another example of a rhyolitic ash flow (Lower Rhyolitic Tuff formation). In the Devil's Kitchen the (Twll Du) in the axis of the Idwal syncline there are bedded pyroclastics overlain by thick pillow lavas. This is clear evidence of the marine environment into which the pillow lavas were extruded during mid Ordovician times.

The mountainous scenery of Snowdonia has been shaped not only by its rock formations but also by glaciation during the Pleistocene Ice Age. An ice cap covered Snowdonia during the last glaciation (Devensian glacial phase 120,000 - 10,000 years ago) and glaciers were nourished in corries near the mountain summits. These glaciers flowed down the pre existing valleys and widened and deepened them into U shaped valleys. Surface rocks were polished and striated by moving ice and much of the eroded material was deposited on the surrounding lowlands as moraines (ridges formed where the glaciers stopped) and glacial till (boulder clay).

It is of historical interest to note that in the early 19th C North Wales was surveyed by some eminent geologists. Professor Adam Sedgwick from Cambridge University defined the Cambrian system and on one of his visits he was accompanied by a young student called Charles Darwin who was eventually to become the famous evolutionist. At the same time Roderick Murchison was working on rocks in Wales which he defined as belonging to the Silurian system. However, the two men argued strongly about the boundaries between the two systems and some 40 years later in 1879 Charles Lapworth settled the controversy by proposing a new system called the Ordovician to cover the disputed rocks between the Cambrian and Silurian.
Here is a brief summary of our field trip on Saturday 2nd July 2016.
We gathered at 10.30am in the car park at Martin's Haven and walked down to the inlet. Here we looked at the basaltic lavas exposed on the beach. The lavas belonged to the Skomer Volcanic Group that were erupted in lower Silurian times (443-428 Ma). They contained many vesicles which would have been filled with volcanic gases when the lavas were molten. Some lavas also showed scoracious or cindery tops indicating that they had been exposed to the air whilst other lavas must have been extruded under water since they showed a pillow structure. At the back of the inlet we discovered evidence of a pyroclastic flow deposit containing fragments of lava set in a fine grained ash.
We walked up on to the Wooltack peninsula, otherwise known as the Deer park to view Skomer Island beyond Jack Sound in beautiful sunshine. Skomer is famed for its puffins and other seabirds but geologically it is formed of lavas similar to those of Martin's Haven. Next we walked around the coast to Renny Slip and back across the barley field to the car park.
A relaxing lunch was taken at the Lobster Pot Pub.
In the afternoon we made our way down the track to Marloes Sands. to examine some of the Silurian sedimentary rocks. Our first stop was in the Gray sandstone Group where we looked at ripple marked limestone with thin shale marking the bedding planes! At Mathew's Slade landslipped strata occurred between two major fault planes. Further along the beach we encountered a basalt intrusion, part of the Skomer volcanics faulted against the Silurian sandstones. Here the lava was vesicular and several veins revealed the green coloured mineral epidote plus numerous calcite veins. Next we observed the sandstone pillars called the Three Chimneys which had been uplifted into a vertical plane by the Caledonian earth movements.  Finally we made it to the Coralliferous Limestone where both colonial and single corals could be seen. Also beds of small brachiopods (Eocoelia) indicated that the sea depth was about 20 metres when these limestones were deposited. In fact researchers have demonstrated how a range of brachiopod species can be used to estimate the approximate depth of the Welsh Basin in Silurian times.
Unfortunately the tide was rising quite rapidly by mid afternoon so we retraced our steps along the beach to Sandy Lane, which is no longer sandy since it has now been surfaced with Carboniferous limestone chippings to confuse the geologists!
Geology / Geology Group Diary (10)
June 09, 2016, 09:31:58 pm
The Geology Group met on Wednesday 8 June at 10.30am at Merlin's Bridge Village Hall.
Here are the notes for the session.

The West Riding of Yorkshire includes most of the Central Pennines that are dissected by several rivers which are tributary to the River Ouse. These rivers flow through and form the Yorkshire Dales from Swaledale in the north; Wensleydale; Nidderdale; Wharfedale; Airedale to Calderdale in the south. For the most part the rocks are Carboniferous Limestone, Yoredale beds and Millstone Grit and they form a faulted tilted structure known as the Askrigg Block formed of Lower Palaeozoic basement rocks. The block dips gently north eastwards but the higher western side is marked by steep scarps along the line of the Dent Fault and the South Craven Fault. The maximum upthrow along the Craven faults is about 1600 metres. Giggleswick Scar (NW of Settle) is a good example of a fault scarp along the line of the South Craven Fault. The younger Namurian gritstones of the Craven lowlands lie on the downthrow side of the fault whilst the older Carboniferous limestone forms the upthrow side of the fault.
The Lower Palaeozoic inliers are small areas of basement rocks (Ordovician and Silurian) that have been exposed by erosion along the north side of the North Craven Fault. The best example is the Ingleton inlier in the valley of the River Twiss where the river cascades over Thornton Force. Here the Carboniferous Limestone (363-325 Ma) rests unconformably on the highly inclined Ingletonian slates (Ordovician, Arenig 493-476 Ma), representing an age gap of at least 110 Ma. The slates originated as Ordovician mudstones that were uplifted, folded and metamorphosed by the Caledonian orogeny, then eroded to an undulating surface over which the early Carboniferous seas deposited a basal conglomerate followed by thick bedded limestones. The Crummack Dale inlier is best known as the source of the Norber erratics. These huge boulders of Silurian gritstone have been plucked by ice from the head of Crummock Dale and deposited on the surrounding limestone surface. The height of the limestone pedestal below the boulder shows the amount of solution weathering that has taken place since the Ice Age.
Malham Tarn also rests on impervious Silurian slates, but notice that its outfall disappears underground when it reaches the limestone at the North Craven Fault. Malham Beck issues as a Vauclusian Spring at the base of Malham Cove, a natural amphitheatre with a well developed limestone pavement above it. The clints (blocks) are cut by rectilinear grykes (fissures) that form as solution weathering widens the joints in the horizontally bedded limestone.
Cawden Hill near Malham village is one of several reef knolls that lie along the southern margin of the Mid Craven Fault. They are formed of calcite mudstones with some shelly limestones rich in corals, crinoids and brachiopods. These reef knolls clearly formed  on the margins of the Lower Carboniferous seas where the water was shallow above the Askrigg Block.
Another spectacular karstic feature is Gaping Ghyll, a large swallow hole that takes the water of Fell Beck off the slopes of Ingleborough. This peak is capped by impervious Millstone Grit and Yoredale beds but as the water drains off on to the underlying limestone it disappears down numerous sink holes (swallow holes) including Gaping Ghyll which has the highest underground waterfall in Britain (98 metres).
The strata in the Yorkshire Dales are gently dipping to the northeast so the Great Scar Limestone is well exposed in the Craven District but farther north the overlying Yoredale beds are more common. Yoredale is an older name for Wensleydale where alternating thin limestones, shales and flagstones occur in rhythmic succession. Such cycles of sedimentation reflect the fluctuating sea levels above the Askrigg Block towards the end of Lower Carboniferous times. Differential erosion of the strata on the valley sides has produced a stepped topography with the limestone forming prominent terraces. Also where the rivers cross the harder rocks there are often spectacular waterfalls. Hardraw Force can be visited through the Green Dragon Inn in Wensleydale and lower downstream on the River Ure are a series of rapids known as Aysgarth Falls.
Swaledale was a major lead-zinc mining area reaching peak production in the late 18th C. The most important mineralised zone occurred on the north side of Swaledale, particularly around Gunnerside Gill. The veins run approximately east-west along pre existing faults in which hydrothermal minerals were precipitated. Gangue minerals such barytes and fluorite also occur in the veins and some old mines have been reworked in recent years in order to obtain these minerals. The source of the hydrothermal fluids is considered to be a large granite intrusion deep below the Askrigg Block and the rising granite also provided the necessary buoyancy to elevate the block above the surrounding lowlands that occupy 'basin' structures where great thicknesses of sediment accumulated in Carboniferous times in contrast to the relatively thin cover over the Askrigg Block.

Geology / Geology Group Diary (09)
May 12, 2016, 10:31:59 pm
The Geology Group met on Wednesday 11 May 2016at Merlin's Bridge Village Hall.

The topic for this month was


This region of Scotland has some of the most spectacular geology and scenery in the whole of the British Isles. It also contains some of Britain's oldest rocks; the Lewisian gneisses that have been radiometrically dated to around 3300 Ma.  This compares with 4000 Ma for the oldest Precambrian rocks in the Canadian Shield. Two geological surveyors, Ben Peach and John Horne, conducted detailed mapping in the NW Highlands in the late 19th century that first revealed the large scale thrust structure of the rocks that were emplaced on the Precambrian foreland.

Lewisian Gneiss.
The highly deformed and metamorphosed rocks are exposed along the coast from the SE corner of Skye to Cape Wrath and in the Outer Hebridean islands of Lewis and Harris. It is considered that these rocks were originally a sequence of magmatic intrusions and that sometime after 3000 Ma they were subjected to intense heat and pressure and converted into coarsely banded gneiss rich in quartz, feldspar and mica. Later around 2200 Ma the rocks were intruded by a series of mafic dykes that are particularly well exposed near the village of Sourie. About 2 kms north of Laxford Bridge on the A838 there are road cuttings where the Lewisian gneiss is cut by sheets of pink pegmatite that pinch and swell indicating that the gneiss was hot and semi molten at the time of the intrusion.

Torridonian Sandstone
The gneisses were uplifted and folded over millions of years and by 1000 Ma a deeply eroded Lewisian landscape had emerged. Then in the late Precambrian the Torridonian sandstone was deposited, resting with a marked unconformity on the eroded Lewisian rocks. The coarse grained red aeolian sandstones are horizontally bedded and have gradually been stripped off  much of the Lewisian basement leaving an exhumed topography with isolated Torridonian mountains such as  Canisp, Suilven, Quinag and Stac Pollaidh The ice scoured surface of the Lewisian gneiss in coastal Sutherland produces a 'knock and lochan' topography (Cnoc is gaelic for small hill) where numerous small lakes occupy glacial hollows that are interspersed with ice moulded hillocks and erratics of Torridonian sandstone.

Cambrian Sediments
Around 544 Ma the region was inundated by the Cambrian sea which laid down the white Eriboll Quartzite unconformably on the eroded Torridonian sandstone surface. The quartzite can be seen in the road cutting at Skiag Bridge where there is also the distinctive 'pipe rock' Here the beds are full of vertical tubes made by burrowing worms (Skolithus), one of the earliest forms of life in the Cambrian. The Durness Limestone overlies the quartzites. It is interesting to note that the trilobite Olenellus lapworthi occurs in the sands and siltstones above the Pipe Rock and this fossil has affinities with North American trilobites. The plate tectonic evidence suggests that NW Scotland was part of Laurentia (NE North America) in Cambrian times and was separated from the rest of Britain by the Iapetus Ocean.

The Moine Thrust Zone
The coastal region of NW Scotland is separated geologically from the rest of the Highlands by a zone of low angle faults known as the Moine Thrust Zone which was formed during the Caledonian orogeny in late Ordovician times. However, in the 19th C geologists were not aware of this structure and so the rock sequence caused major problems because it appeared that the unaltered Cambrian sediments passed conformably upwards into high grade schists.  The Law of Superposition indicates that the schists must be the youngest rocks in the sequence, but it was hard to explain how unaltered Cambrian sandstones and limestones could be overlain by highly metamorphosed schists. There was much controversy as to how this sequence could have formed. In 1883 Archibald Geikie, Director of the Geological Survey sent two experienced surveyors (on horseback) to this wild remote area of Scotland to carefully map the rock outcrops. As a result Peach was able to demonstrate that the eastern Moine schists (late Precambrian in age) had been thrust westwards over the rigid foreland block of Lewisian gneiss, Torridonian sandstone and Cambrian sediments by a series of large scale low angle thrust faults.(Moine, Glencoul and Sole thrusts). The thrust planes are separated by smaller imbricate faults and duplex structures. Thus we see how the older schists were moved many miles by major thrusts and placed on top of younger Cambrian rocks! A visit to the nature reserve at Knockan Cliff shows the position of the Moine Thrust which is marked there by a fine grained flinty rock called mylonite, produced by rock grinding along the thrust plane.

Geology / Geology Group Diary (08)
April 14, 2016, 08:56:00 pm
The Geology group met at 10.30am at Merlin's Bridge Village Hall on Wednesday 13 April 2016.

Here are the notes for the topic 'GEOLOGY OF SHROPSHIRE'

The oldest rocks in Shropshire are of late Precambrian age. The Uriconian volcanics (566 Ma) form a line of hills extending from the Wrekin south westwards to Caer Caradoc.  These volcanic andesitic lavas were formed in an island arc complex (similar to the Caribbean islands today) above a subduction zone where oceanic crust was forced down at a plate margin. Also rhyolitic ash flows were produced by explosive eruptions from volcanoes lying along the line of the present Church Stretton Fault.

Adjacent to the Uriconian volcanics are the Precambrian Longmyndian sediments comprising up to 8000 metres of sandstones and shales forming a deep syncline with an inverted western limb. The higher parts of the sequence contain clasts of Uriconian lavas so these sediments must have been deposited in the shallow seas surrounding the volcanic islands. There is a major unconformity between the eastern (older) and western (younger) Longmyndian sediments followed by a period of intense folding and faulting during the late Precambrian. A good locality on the eastern side of the Longmynd is the Carding Mill Valley near Church Stretton where you can examine the steeply dipping mudstones and sandstones that contain various sedimentary structures including ripple bedding, graded bedding and also bands of volcanic ash. These were deposited under shallow marine conditions where volcanic ash would be erupted from time to time during the later phases of Uriconian vulcanicity. Some of the shales contain fossilised rain spots which were probably preserved as the mud flats dried out along the sea shore.

The Ercall is a small hill on the north east side of the Wrekin. It is significant in that the unconformable junction between the Precambrian igneous rocks and the basal Cambrian sediments is well exposed in the old quarries.  About 560 Ma the Uriconian lavas of the Ercall were intruded by a large mass of granophyre; this is a type of pink granite formed mainly of quartz and orthoclase feldspar. However, banked up against the granophyre are the basal Cambrian sediments known as the Wrekin Quartzite. Since the Cambrian period began around 545 Ma the unconformity represents a time gap of at least some 15 million years. Quartzite is the term used for a metamorphosed sandstone but here the Wrekin quartzite is simply a quartz rich sandstone with a conglomerate at its base. It shows extensive ripple bedding indicative of shallow marine conditions and many of the pebbles in the basal conglomerate are of locally derived rhyolite, tuff and granophyre. The Cambrian shoreline must have surrounded the Precambrian volcanic island arc.

The Shelve inlier provides a useful sample of Ordovician rocks (495 - 443 Ma) in Shropshire. The inlier lies immediately west of the Longmynd and the Pontesford- Linley Fault. The oldest formation is the Stiperstones Quartzite that dips steeply westwards and forms a spectacular ridge of frost shattered tors developed under periglacial conditions at the end of Pleistocene times. The quartzite is a shallow water deposit that is an exceptionally pure white sandstone with a conglomeritic base. The succeeding Mytton Flags are over 900 metres thick; they are hard, blue-grey flaggy siltstones that display conspicuous rectilinear jointing. These joints formed as the sediments dried out. Geologists consider that during late Devonian times a deep seated granite pluton was the source of hydrothermal fluids that were injected into the joints of the Mytton Flags precipitating minerals such as galena (PbS), sphalerite (ZnS) and barytes (BaSO4). Snailbeach mine was a major producer of lead ore from1845 to 1913.

Wenlock Edge is formed of a sequence of limestone and shale deposited in mid Silurian times (430-411 Ma). This is an area of classic scarp and vale topography where resistant limestone alternates with softer shale. You will see on the cross section that the Wenlock Limestone forms a scarp overlooking Ape Dale (Wenlock Shale) and the Aymestry Limestone forms a scarp overlooking Hope Dale (Lower Ludlow Shale). The limestone is  full of coral reefs that were formed  in warm shallow seas around 25°S. The bedded limestone wraps round the reefs which are mainly built of corals, sponges and bryozoans, but there are also numerous brachiopods, trilobites and crinoids in the limestone. Wenlock limestone has been exploited as a flux for iron smelting since Abraham Darby first developed coke fired blast furnaces in Coalbrookdale near Ironbridge in the early 18th C. Much later  iron smelting became established in the Dudley area (40 km east of Wenlock Edge) where  the underground mining of  limestone took place. Victorian fossil collectors obtained some beautiful specimens from the Dudley caverns including the famous 'Dudley bug', a trilobite with the scientific name of Calymene blumenbachi.

The Ironbridge Gorge on the River Severn is an example of a glacial overflow channel. At the end of the Pleistocene Ice Age some 15,000 years ago a proglacial lake known as Lake Lapworth occupied much of the lowland north of Telford. The lake was impounded between the retreating ice front to the NW and higher ground to the SE. The preglacial Severn drained northwards into the Dee estuary but as Lake Lapworth began to overflow, it cut two outlets, one at Newport and the other at Ironbridge.  Eventually, the former outlet was abandoned as the Ironbridge gorge was deepened and the Severn drainage was diverted permanently to the Bristol Channel. As the lake was drained several metres of lacrustrine silts and clays were laid down such as those exposed just to the north of Wellington and fluvio glacial sands and gravels were deposited near the entrance to the Ironbridge gorge.

Geology / Geology Group Diary (07)
March 09, 2016, 08:57:59 pm
The Geology Group met at Merlin's Bridge Village Hall at 10.30 am on Wednesday 9 March. Here are the notes for the topic.
The Lake District National park in Cumbria was created in 1951 in recognition of its outstanding natural landscape sculptured from Palaeozoic rocks by the forces of weathering and erosion. Geologically the region can be divided into an older Lower Palaeozoic inlier surrounded by a younger Upper Palaeozoic margin.
Evolution of the Central Lake District; the Lower Palaeozoic inlier.
.The oldest strata in the central Lake District belong to the Skiddaw Group which are of Lower Ordovician age (Arenig 485-470 Ma). These were deposited on the floor of the Iapetus Ocean which separated what is now Scotland, Greenland and Labrador from NW Europe. This wide subsiding geosyncline received vast amounts of sand and mud throughout Arenig times which eventually produced the Skiddaw slates, a sequence of turbidites that have been deformed, folded and cleaved during the Caledonian orogeny.  The graptolitic fauna of these rocks also clearly indicates the deep water marine conditions which existed at this time. However, as the Iapetus Ocean began to close, subduction was taking place below the Solway trench and magmas were generated in the subducted oceanic crust. These magmas fed the granitic intrusions which in turn produced the Eycott and Borrowdale volcanoes that built up some 4000 metres of andesitic lavas, tuffs and agglomerates belonging to the Borrowdale Volcanic Group. No marine fossils have been found in these rocks, confirming their terrestrial origin. Such volcanic rocks are resistant to erosion and form the highest and most rugged mountain scenery in the Central Lake District. By the end of Ordovician times (443 Ma) volcanic activity had almost ceased and a marine transgression occurred depositing thin calcareous mudstones which are known as the Coniston Limestone Group. The narrow outcrop of these rocks forms an important marker horizon separating the rugged Borrowdale Volcanic country from the more subdued topography of the overlying Silurian rocks to the south. These sediments of Silurian age (443-417 Ma) consist of some 3500 metres of mudstones, sandstones and greywacke that were deposited by turbidity currents in a vast subsiding trough.
At the end of this long period of Silurian sedimentation came the Caledonian Orogeny when continental collision and mountain building occurred. The Laurentian continental plate moving SE collided with the NW European landmass. Ordovician and Silurian sediments and volcanic tuffs were strongly folded and deformed with the predominant Caledonian structural trend aligned NE-SW. Platy minerals were reoriented to produce slates with strong cleavage often at a high angle to the original bedding. By the early Devonian (395 Ma) a huge batholith was emplaced below the Lake District and the Northern Pennines. This batholith has been exposed by erosion in a few localities as in the Shap; Skiddaw and Eskdale granites and the Carrock Fell complex. However, its overall presence has been proved by geophysical surveys (positive gravity anomalies) and a borehole in Weardale. The heat generated by these plutonic intrusions has resulted in the thermal metamorphism of the surrounding rocks and also in the formation of mineral veins from rising hydrothermal fluids.
The Upper Palaeozoic Margins of the Lake District 
During the Devonian period (417-354 Ma), erosion reduced the Caledonian mountains to a peneplain and some of the waste  products of this erosional process known as molasse deposits can be seen in the Mell Fell conglomerates on the north side of Ullswater. Then by the beginning of the Carboniferous marine transgression deposited a sequence of limestones and shales. These Carboniferous strata lie unconformably on the eroded  Lower Palaeozoic rocks. There is a time gap of nearly 50 Ma between the end of the Silurian and the deposition of the first Carbonifeous strata. This unconformity is well displayed in a stream section near the Shap Wells hotel where steeply dipping Silurian slates are overlain by horizontal basal Carboniferous sediments which consist of a pinkish conglomerate containing distinctive orthoclase crystals of Shap granite. Since the Carboniferous beds could only have been formed after the granite had been intruded, exposed and eroded; the granite itself must be of Devonian age. This is confirmed by isotopic dating which puts the granite intrusion at 393 Ma and the basal Carboniferous at 350 Ma.
Upper Carboniferous coal measures are preserved in NW Cumbria around Whitehaven but these would have been more extensive before the Variscan earth movements further uplifted the Lake District, downfaulted the Vale of Eden and activated the Pennine fault system. Continental desert conditions were then established over the region in Permian times (290-252 Ma) as shown by the dune bedded sandstones exposed around Penrith. On the west coast around St Bees Head  the Permo-Trias contains evaporates deposited in shallow water desert basins.
Quaternary Events
One of the most outstanding aspects of the scenery of the Lake District is the radial drainage pattern. The lakes and rivers all flow outwards from the central mountain hub like the spokes of a wheel. This drainage pattern is thought to have originated on the domed surface of post Devonian rocks that covered the region. Eventually, as erosion removed most of the cover rocks (except for the margins) the radial drainage pattern was superimposed upon the underlying Lower Palaeozoic inlier. This explains why the present superimposed drainage system bears little relation to the geological outcrops.
Finally, the Pleistocene glaciation (2 Ma - 10,000 BP) resulted in the sculpturing of the mountain core as the Lake District ice cap expanded and ice moved outwards down the radial valleys. Hence we see corries where glaciers were nourished, U shaped valleys eroded by ice and glacial till (boulder clay) deposited in the surrounding lowlands where ice sheets melted. Today the processes of weathering and erosion continue to mould the landscape.
There will be a U3A geological field trip on Saturday 7 May 2016
Meet at Stackpole Quay NT car park at 10.30 am
In the morning we will be looking at the Stackpole wrench fault and at folds & fossils in the Carboniferous Limestone.
Lunch in the Boathouse Tea room.
In the afternoon we drive to Freshwater East to examine the Upper Silurian beds and the Old Red Sandstone. There will be a walk across the sandy beach ..about 1 km but very flat.
Low tide 13.37hrs BST
Geology / Geology Group Diary (06)
February 11, 2016, 10:36:31 pm
The group met on Wednesday 10 February at 10.30 am in Merlin's Bridge Welfare Hall.

The Tertiary period began around 65 Ma and continued up to 2 Ma.  During the break up of the super continent Pangaea, the North American plate and the Eurasian plate rifted apart to form the North Atlantic Ocean. In Palaeocene times (65-56 Ma) Britain was situated above a mantle plume (sometimes referred to as a hot spot) where the earth's crust was under stress from plate rifting. As a result fissures opened up along a line from Ireland to the Hebrides from which basaltic lavas were extruded and igneous centres developed, surrounded by ring dykes and cone sheets and radiating dyke swarms. A vast lava field (1.8 million km2) called the Thulean Plateau extended from West Scotland  through the Faroes, to Iceland and Greenland.
The Plateau Basalts
Up to 2000 metres thickness of basalt lavas were erupted in Antrim, Mull and Skye during the Palaeocene. Even today after extensive erosion, the lavas are 1800 metres thick on Mull. The individual lava flows give rise to stepped topography which is caused by the weathering back of the softer vesicular slag at the top of each flow. Between the lava flows there are often found red lateritic soils. These iron rich deposits contain plant remains, particularly in the leaf beds of SW Mull where a temperate flora of ginkgo, plane, hazel and oak occur. The leaves are well preserved and are thought to have fallen into a shallow lake. In 1819 a magnificent specimen of a tree trunk was discovered embedded in lavas in the cliffs of western Mull; it is known as MacCulloch's tree!
The basalt lavas of the Giant's Causeway and the Isle of Staffa are world famous for their hexagonal columnar jointing. The columns result from the cooling and contraction of the lavas under perfectly stable conditions and they form perpendicular to the base and surface of the flow. The top of the lava flow would be filled with escaping gases which produced a vesicular slaggy texture. On the island of Eigg the lava cooled so quickly that it produced a glassy rock called pitchstone (similar to obsidian) that forms the Sgurr of Eigg.
The Igneous Complexes
There are several major plutonic centres extending northwards from the Mourne Mts in Northern Ireland through the Western Isles of Arran, Mull, Ardnamurchan, Rum and Skye. These centres represent the remains of Tertiary volcanoes and their underlying magma chambers.
On Skye the magma was emplaced in a sequence of Precambrian rocks (Lewisian gneiss and Torridonian sandstone) overlain by basalt lavas. The magma was rich in mafic minerals (plagioclase, pyroxene and olivine). Layered gabbros were produced by the settling out of denser minerals and basaltic volcanoes erupted at the surface. Today we see the Cuillin Hills formed from the deeply eroded gabbronic magma chamber. However, the nearby Red Hills are formed of granitic magma produced within the same igneous complex. The granites were probably formed by the melting of the Precambrian basement rocks. In the Cuillins many minor intrusions occur as cone sheets rising out of the volcanic conduit and sloping outwards in a circle around the volcanic centre.
The island of Mull is almost entirely formed of an extinct volcano and its eroded magma chamber with three major eruptive centres within the complex. Numerous cone sheets form a distinctive arcuate pattern around central Mull. However, the Mull volcano is also characterised by the presence of ring dykes. These are cylindrical intrusions produced by magma being forced up through ring faults surrounding a central area of subsidence. As magma is removed from the magma chamber the centre of the complex sinks creating a cauldera of subsidence. Think of a full wine bottle as a magma chamber and the cork representing the overlying rocks. Force the cork downwards and wine will be forced upwards around the cork creating a 'ring dyke'.
Dyke swarms occur throughout the Tertiary volcanic province and they all demonstrate a NW-SE trend. They are thought to be associated with shearing stresses produced by the opening of the North Atlantic during the early Palaeocene. Some 300 dykes can be seen on the south coast of Arran and these represent a7% crustal extension in a 20 km section. Perhaps the most famous Tertiary dyke is the Cleveland dyke in NE England. This can be traced for 400 kms from Mull to North Yorkshire and is thought to have been emplaced as a single pulse within a few days!
Geology / Geology Group Diary (05)
January 14, 2016, 05:09:13 pm
The Geology Group met at 10.30 am on Wednesday 13 January at Merlin's Bridge Community Centre.
Here are the notes for the session on the topic:
The Vale of Severn is for the most part formed of Lias Clay (Lower Jurassic 205-180 Ma) and one of the best places to examine these rocks is at Hock Cliff near Fretherne on the shores of the River Severn. Here bands of argillaceous (clayey) limestone reinforce the thin muddy shales. These strata were deposited in a shallow muddy sea in which belemnites, ammonites and sea lilies (crinoids) were flourishing, but the most famous fossil found at Hock Cliff is Gryphaea arcuata, an extinct type of oyster , also known as the 'Devil's toenail'. The Lias Clay was used extensively for brick making in the 19thC and many small  brick works' quarries have yielded a wide variety of fossils. Blockley Quarry near Moreton in Marsh is a well known fossil collecting site. Ammonites such as Liparoceras cheltiense, and bivalves
The low undulating clay vale is in places punctuated by small hills known as outliers, the most famous of which is Bredon Hill. Outliers are small areas of newer rock surrounded by older strata; for example, Bredon Hill is capped by Inferior Oolite but surrounded by Lias Clay. An outlier is really an isolated outcrop left behind as the main escarpment gradually retreats under the influence of weathering and erosion. A closer look at Bredon Hill reveals that it has a steep scarp to the north and the strata dip south with a strong fault along its southern margin. Notice the stepped profile, well seen on the eastern side of the hill. This bench feature is produced by the resistant marlstone, a ferruginous sandstone at the top of the middle Lias.
The Cotswold escarpment is formed mostly of  limestones of the Inferior Oolite Group (180 - 169 Ma). These limestones are formed of tiny ooliths around which calcium carbonate has been deposited by currents in shallow shelf seas (similar to the Bahamas shelf today). Leckhampton Hill near Cheltenham shows a section through the Inferior Oolite. The main limestone is well bedded and jointed and so makes an excellent building stone since it can be cut easily into blocks. Much of Regency Cheltenham was built of stone from the Inferior Oolite. The Devil's Chimney was left behind by the quarrymen in the 1780s to form a local land mark. Crickley Hill provides an example of a promontory on the Cotswold scarp which it is cut by a deep re-entrant valley followed by the A417 Gloucester road. The cliffs along the face of Crickley hill are formed of the Pea Grit; this is a pisolitic limestone where the grains are much larger than the ooliths in the freestones. Clypeus (echinoid), Trigonia (oyster) and rhynconellids (brachiopods) are common fossils. Highly developed ammonites with intricate suture patterns are also found in the Inferior Oolite.
The Great Oolite Group (169 - 159 Ma) overlies the Inferior Oolite and is found mainly on the dip slope of the Cotswolds. Near the base of the Great Oolite is a famous bed called the Stonesfield Slate. This is not a slate in the geological sense but a sandy limestone that is thinly bedded and so forms flagstones 2-3 cm thick. These were extensively quarried for roofing tiles in the 18C & 19C and can be seen on old cottages and farm buildings throughout the Cotswolds today. However, as long ago as 2000 BC in the Neolithic period  the 'slates' were used to build the entrance to the Bellas Knap long barrow! But the Stonesfield Slate was made famous by William Buckland, the great Oxford geologist who first identified the remains of the carnivorous dinosaur Megalosaurus bucklandii in 1827. There are also leaf impressions of cycad leaves indicating that a rich vegetation must have bordered the coastal lagoons in which the sandy limestones were deposited.
At the top of the Great Oolite is the White Limestone Formation which can be seen in the old cement works quarry at Kirtlington, a few miles west of Oxford. Here the limestone is hard, white and shelly. The most abundant brachiopod is Epithyris oxonica which makes up much of the rock. However the quarry has also yielded the bones of pterosaurs (flying reptiles), plesiosaurs (marine reptiles) and early mammals.
The Cotswolds provide a good example of scarpland topography where relatively resistant limestones alternate with softer clays and shales. The clay vale, the scarp and the dip slope are the three essential features of this type of topography. Note that the strata in the Cotswolds dip gently south east and young in the direction of dip. Since the limestone is permeable, water percolates down to the level of the underlying impermeable clay thus producing springs at the junction of the two rocks. Many villages developed in Anglo Saxon times along the spring line at the foot of the Cotswold scarp. Dry valleys are also common in the limestone since the water table has been gradually lowered as the scarp has receded.

Geology / Geology Group diary (04)
December 10, 2015, 05:40:35 pm
The Geology Group met on Wednesday 9 December. These are the notes for the topic 'Geology in the Weald of Kent & Sussex'.

This region of South East England is formed of Cretaceous strata, although the Purbeck beds underlie the surface rocks in the central Weald. Structurally, the Wealden uplift is due to the Alpine earth movements that produced an anticline which dips gently away from its east-west axis, although the fold also plunges to the west.

The High Weald is formed of the Hastings Beds (145 - 137 Ma) which are the oldest Cretaceous rocks in the centre of the anticline. The rocks are mainly sandstones that show cross stratification, ripple marks and plant remains. These were deposited in deltas within the shallow waters of the Wealden Lake. On the shores of the lake grew conifers, cycads and giant horsetails on which dinosaurs such as Iguanadon were feeding. Also there are clay bands containing ironstone nodules, the source of ore for the Wealden iron industry which developed in the 16th C to provide cannon for Tudor ships that were build of oak from the Wealden forests. The area was extensively forested until the demand for charcoal for smelting and timber for ships led to the removal of much of the woodland. Hammer ponds were used to power water wheels that operated bellows and hammers for the iron works. The High Weald was the centre of the iron industry in Britain until the beginning of the industrial revolution when Abraham Darby developed the coal fired blast furnace in 1709 in Coalbrookdale. There were also numerous small quarries that provided local building stone. Bateman's House, formerly Rudyard Kipling's home, is a good example of the use of local Ashdown sandstone. The High Weald forms a major watershed separating north and south flowing rivers including the Medway and the Wey, the Arun, the Ouse and the Cuckmere. River capture commonly occurs where some rivers cut back by headward erosion and divert the headstreams of others, thus increasing their drainage system.
The Low Weald is formed of Weald Clay (137 - 130 Ma) and forms a horseshoe shaped outcrop around the High Weald. The clay vales are poorly drained (impermeable clay) and mainly provide pastureland. Brick making was based on the Weald Clay, particularly during the 19thC bricks were in demand for the London market.

The Lower Greensand (130 - 125 Ma) lies above the Weald Clay and it has been worn back by erosion to form prominent scarps overlooking the clay vales. Leith Hill on the northern escarpment forms the highest point in the Weald. The sandy acidic soils generally support heathland and coniferous woodland. A spring line marks the base of the greensand where water emerges along the junction with the clay. Note that the greensand is commonly red in colour but the name came from green sands containing glauconite ( an iron mineral) that outcrop on the Dorset coast. The Lower Greensand was deposited under shallow marine conditions as the Wealden Lake was invaded by the sea. Later as the sea deepened the Gault Clay (125 - 100 Ma) was laid down. The clay is one of the most fossiliferous horizons in Britain containing a rich marine fauna of ammonites, bivalves and gastropods. It forms a narrow vale at the foot of the chalk escarpment. At Folkestone Warren rotational landslipping occurs in winter when the overlying chalk is saturated and slides over the impermeable Gault Clay. In 1915 a passenger train was derailed on the coastal railway line which was buckled by a landslide.

The Chalk (100 - 65 Ma) encloses the Weald on three sides forming inward facing escarpments along the North and South Downs. The present river system was initiated on the chalk cover which has since been eroded over the Weald. The rivers have cut gaps through the chalk scarp; for example, the Wey gap at Guildford and the Ouse gap at Lewes. Note that where the dip of the chalk is steep as on the Hog's Back, the outcrop is narrow. Where the dip is gentle or horizontal, the outcrop forms extensive undulating downland. Dry valleys are common where the water table has been lowered. The Seven Sisters on the Sussex coast are dry valleys truncated by the sea. A spring line occurs at the base of the scarp along the junction between the chalk and the underlying Gault clay. Anglo Saxon settlements developed along the spring line. Villages with suffixes such as 'ham, ton and ing' date back to this period.
Geologically, the chalk is a fine grained white limestone formed from calcareous mud containing microscopic coccoliths derived from marine plankton. The Cenomanian transgression in late Cretaceous times covered much of southern England and since there was little sand and mud brought down by rivers, the sea remained relatively clear and the chalk sediment was free from impurities. Fossils are fairly common in the Lower Chalk including ammonites, belemnites, echinoids, bivalves and brachiopods. Micraster, the heart urchin is common as are brachiopods such as terebratulids and rhynchonellids. The Upper Chalk is characterised by the presence of flint nodules which may have been precipitated from silica rich ground waters percolating through the chalk. However, recent research suggests that the flint was formed by the sub surface breakdown of siliceous organisms such as sponges, radiolaria and diatoms during the deposition of the chalk.
Geology / Geology Group Diary (03)
November 11, 2015, 07:44:45 pm
Geology Meeting 11 November 2015  Notes on the geology of the Peak District.


The Peak District is the name given to the upland region of the Southern Pennines that extends through Derbyshire into North Staffordshire. The region was designated as Britain's first National Park in 1951.  Geologically the Peak District is formed of an eroded anticline the core of which is made of Carboniferous Limestone. On the flanks of the anticline is the Millstone Grit overlain by Coal Measures.


Miller's Dale and Monsal Dale through which flows the River Wye are classic steep sided wooded valleys cut into the Carboniferous Limestone. At Hobs House near the Monsal Viaduct, the limestone stacks yield a variety of corals including the colonial Lithostrotion and the solitary coral Dibunophyllum.  Brachiopods such as Productids and Spiriferids are also common in the reef limestones around Castleton, providing evidence of the existence of warm shallow seas in Lower Carboniferous times. These limestones have been subject to karstic weathering where deep potholes and caves have developed due to the solution action of rain water percolating through fissures and joints. Winnat's Pass is an example of a collapsed limestone cave system. The Blue John caverns are also well known for their purple coloured fluorspar crystals. Fluorite is a gangue mineral in lead zinc veins in the Pennines, precipitated from hydrothermal fluids.
In the southern part of the region the limestone is dissected by south flowing rivers in Dovedale and the Manifold valley where there are numerous features such as caves, potholes and reef knolls. The wooded valleys are cut into the bare upland plateau where surface drainage is minimal and dry valleys are commonplace.
Volcanic activity during Lower Carboniferous times is recorded by several lava flows in the Wye Valley and also at Calton Hill near Buxton. Here, in what is now a disused quarry, the volcanic vent marks the site of an olivine basalt extrusion which shows excellent columnar jointing. Above the columnar basalt there are several contorted scoriaceous flows which appear to issue from several small vents.  The cindery surface is similar to present day Hawaiian aa lavas and the flows contain pyroclastic material such as blocks of columnar basalt and vesicular lava. The resulting agglomerate may represent fall back debris in the vents.  This SSSI is unique in that the lava contains olivine nodules (xenoliths) derived from peridotite in the upper mantle.

THE DARK PEAK; MILLSTONE GRIT (Namurian age 325 - 315 Ma)

The Castleton area shows the contrast between the light grey reef limestones and the dark overlying shales and gritstones of Namurian age. Mam Tor about 2 kms west of Castleton,
is also known as the 'Shivering Mountain' because its south east face has been subject to numerous rotational landslips caused by waterlogged shales underlying thin sandstones. The A625 Sheffield to Chapel en le Frith road used to run along the foot of Mam Tor a section of it was permanently closed in 1979 due to continued slippage.
Millstone Grit forms much of the high moorland in the Peak District. Kinderscout is a good example of gritstone scenery with roughly horizontal beds of coarse sandstone alternating with shales. The gritstone forms steep scarped 'edges' (escarpment) where the strata are exposed around the margins of the Kinderscout plateau. The Pennine Way starts at Edale and can be followed for 268 miles to Kirk Yetholm in the Scottish Borders. From Edale the well worn track leads up Grindsbrook Clough to the southern edge of Kinderscout where the gritstone is covered by thick peat covered moorland. Numerous 'mushroom' rocks stand sentinel above the escarpment. They have been carved by wind and rain into varied shapes as the joints have been widened leaving the harder gritstone standing proud.
The Roaches in North Staffordshire are located in the SE corner of the National Park on the Leek - Buxton road. The structure is synclinal with outward facing gritstone scarps. There is a shallow coalfield within the syncline (Goldsitch Moss). Here the Millstone Grit shows excellent cross stratification, evidence of its deltaic origin when large south flowing rivers brought down vast amounts of sandy sediments during Namurian times.

John Souster

Some load themselves like sherpas
Upon the Pennine Way,
And bowing under bulging packs
With downcast eyes and doubled backs
They trudge along unheedingly,
Indifferent to the scenery
Upon the Pennine Way.

But let me travel lightly
Upon the Pennine Way.
I'll walk alone, with time to see
The berries on the rowan tree;
To see the fern beside the fall
And listen to the curlew call;
To notice where the glaciers flowed
And where the clanking cohorts strode
Upon the Pennine Way.

Yet each must manage somehow
To walk his Pennine Way,
And all will find it rough at times
And none avoid some weary climbs.
But when it draws to close of day,
And when the maps are put away,
And when we sit and journey back
In memory along the track,
Then not the one with speed his boast,
And not the one who carried most
Will finish wisest of the host
Who tread their Pennine Way.