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Geology / Geology Group Diary (34)
September 13, 2018, 06:16:37 pm
The Geology Group met at Merlin's Bridge village hall at 10.30am on Wednesday 12 September 2018. The topic for this month was BRITISH JURASSIC GEOLOGY.

A shallow sea on the western margin of the Tethys Ocean spread across Britain during the Jurassic Period (201-145 Ma) when the area lay between 30° and 40° N. A succession of alternating marine clays and limestones were laid down but due to several axes of uplift the Jurassic strata is thinner over the swells and much thicker in the intervening basins. The three most prominent uplifted areas of pre Jurassic rocks are the Mendip, Moreton and Market Weighton axes. Outcrops of Jurassic rocks extend in a diagonal belt from the Dorset coast up through the Cotswolds to the North Yorkshire Moors. In the early 19th C William Smith, a canal engineer, was the first to work out the Jurassic sequence of strata using ammonites as zone fossils. In 1815 he produced the first geological map of England and Wales.
The Lias clays are exposed in the cliffs around Lyme Regis and Charmouth in west Dorset. During the 19thC  quarrymen used the term Lias  (a corruption of the word layers) to describe the sequences of alternating thin limestones and bluish grey clays that reach a thickness of over 130 metres. They represent the sediments deposited on the margins of the Tethys Ocean which spread over much of the Permo-Triassic landscape of Britain. The Lias sea contained a rich fauna of ammonites and marine reptiles such as Ichthyosaurs and Plesiosaurs made famous by Mary Anning, the Victorian fossil collector who lived in Lyme Regis. Dark bituminous shales also occur in the Lias which suggest that the sea deepened from time to time. The shales contain iron pyrites, a mineral formed where there is a lack of oxygen in deep waters. The Bridport Sands occur at the top of the Lias sequence and mark a change in environmental conditions as sandy sediment was washed into the area.
Liassic sediments also outcrop along the coast between Staithes and Robin Hood's Bay in East Yorkshire.  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!
MID JURASSIC (174-163 Ma)
The Cotswold Hills overlook the low undulating Vale of Severn that is punctuated in places  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 that are formed of tiny ooliths consisting of concentric layers of calcium carbonate that has been deposited by circulating currents in shallow shelf seas (similar to the Bahamas shelf today). The colour of the limestone varies from pale buff to orange brown depending on the concentration of iron minerals in the cement. 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; hence it is referred to as a freestone. Much of Regency Cheltenham was built of stone from the Inferior Oolite. The Devil's Chimney at Leckhampton 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  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
The Dorset coast between Weymouth to Swanage provides some excellent exposures of Upper Jurassic rocks. The Oxford Clay outcrops in the core of the Weymouth anticline but it extends in a broad swathe  north west through Oxford to Lincolnshire and beyond. It has yielded aquatic reptilian remains, and a variety of fossil ammonites, belemnites and brachiopods.  Economically it is important as a brick making clay which is high in carbon content thus reducing firing costs. Fletton bricks from Peterborough were used in building much of 19th C London.
On the coast around St Alban's Head the Kimmeridge Clay appears below the Portland Stone cliffs. Kimmeridge Bay is the type locality for this formation, consisting mainly of black shales and clays with bands of limestone that form the Kimmeridge ledges. Some of the shales are bituminous and combustible; in fact, the Kimmeridge beds are an important source rock for petroleum under the North Sea. Wytch Farm on the Isle of Purbeck is the largest onshore oil field in Britain
The Isle of Portland forms the southern flank of the Weymouth anticline and it is highest in the north and dips gently down to sea level at Portland Bill in the south. Although the Portland Stone extends across most of the area 75% of it is covered by the lower Purbeck Beds (quarry overburden). The Portland Stone is a fine grained white limestone that is well jointed and easily dressed into rectangular blocks. It became popular as a building stone in the 17th C when St Paul's Cathedral was rebuilt after the Great Fire of London. Most of the quarries are on or near the coast so that the stone could be exported by sea. A fossiliferous limestone known as the Roach occurs at the top of the Portland Stone and contains 'Portland screws and osses 'eds ' otherwise known as Aptylexia (gastropod) and Laevitrigonia (bivalve). Large ammonites such as Titanites have been extracted from the Portland beds.
The coast from Durdle Door to Lulworth Cove and Mupe Bay is formed of a wall of Upper Jurassic strata that create a barrier protecting the softer Wealden Beds (Lower Cretaceous) from the continuous erosion of the sea which has already broken through in several places. Stair Hole adjacent to Lulworth Cove provides a good example of the way that the sea is excavating the soft Wealden beds behind the Portland/Purbeck wall. The famous Lulworth crumple in the Purbeck beds shows the effect of the Alpine earth movements during the Oligocene. Lulworth Cove itself has been cut back through the sands and clays of the Wealden beds to the high cliffs of the massive Chalk ridge that extends eastwards into the Purbeck hills. Note the exposure of Upper Greensand (at the base of the chalk) near where the road reaches the cove and here the greensand really is greenish in colour due to the presence of the iron mineral glauconite. On the east side of Lulworth Cove is a fossil forest that consists of silicified boles which mark the base of conifers that grew during Purbeck times.
On the Isle of Purbeck the most important structural feature is the Purbeck monocline produced by the northward thrust of the Alpine orogeny. The chalk ridge of the Purbeck hills can best be seen around Corfe Castle that stands on a knoll between a twin water gap. Here the chalk is almost vertical hence the width of the Purbeck hills is relatively narrow. (Note the rule of thumb is that the steeper the dip of a stratum, the narrower the outcrop). The underlying Wealden, Purbeck and Portland beds all dip steeply north near Corfe but then become horizontal across the Isle of Purbeck forming the southern limb of the monocline. The best place to see the actual curvature of the monocline is in St Oswald's Bay to the west of Lulworth.
Swanage was the centre of the Purbeck stone trade in the 18th & 19th C but in Medieval times Purbeck 'marble' was much in demand  for cathedral interiors, eg.Westminster Abbey and Salisbury Cathedral. A beautiful example of 'marble'columns can be seen in Eldon Memorial Church near Kingston.
John Downes
Geology / Re: Geology Group Diary (33)
July 12, 2018, 03:26:44 pm
6 more images of Permo Triassic.
Geology / Geology Group Diary (33)
July 12, 2018, 03:19:59 pm
The Geology Group met at 10.30am on Wednesday 11 July at Merlin's Bridge village hall. This month's topic was...
The Permian Period lasted from 290 to 245 Ma which brought to an end the Palaeozoic era. The period was terminated by a major catastrophic event known as the Permian Mass Extinction when 95% of all animals and plants became extinct.  Invertebrate groups such as trilobites, goniatites, rugose corals and many brachiopods became extinct. On land many amphibians and reptiles died out to be replaced by the early dinosaurs in the succeeding Triassic Period (245-208). A combination of various factors may have brought about the great extinction. Here are some of the possible events. 1.Asteroid impact; dust and rock fragments from the collision of an asteroid 100 km in diameter would blot out sunlight for years. However, it is difficult to find evidence of such craters since the earth's surface has changed considerably since Permian times. 2.Formation of Pangaea; this super continent emerged during the Permian causing a eustatic (world wide) lowering of sea level and marine regression over the continental shelves thus destroying shallow water marine habitats. Polar glaciations would also contribute to lowering sea level. 3.Vulcanism; the vast Permian basalt lava flows in Siberia would have been produced by powerful eruptions that would emit CO2 and SO2 in enormous quantities causing climate change and the destruction of vegetation and consequent death of animal life.  4.Climatic events; Global warming due to enhanced greenhouse effect would cause increased aridity (continental climate) and salinity (high evaporation), severe decline in oxygen levels and the presence of excess carbon dioxide, methane and hydrogen sulphide in the oceans would poison marine life.
The Permo-Triassic Environment.
[The Permo-Triassic rocks are often referred to as the New Red Sandstone]
As the Rheic Ocean closed and the Variscan Mountains were formed, the supercontinent of Pangaea developed as the existing continents collided. Britain was located between 20° and 30°N on the north west side of the Tethys Ocean. At the beginning of Permian times Britain was an arid upland area where barchan sand dunes accumulated under the influence of the north east trade winds. Fault bounded basins such as the Vale of Eden received great thicknesses of aeolian (wind blown) sediment at this time. In the late Permian the marine transgression of the Zechstein Sea covered much of NE England and the North Sea area. The sea initially covered the desert landscape with muddy sediments forming 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 rapid evaporation occurred in the hot arid climate within the Zechstein Sea, a sequence of repeated cycles of desiccation 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 desiccation. In the 1970s the Boulby mine near Staithes (Cleveland) was opened in order to extract potassium chloride (KCl)  and halite (NaCl) from up to 1500 metres below the surface. The potash is required for agricultural fertilizer; rock salt is also produced and both minerals are exported via Teesside. North Sea oil and gas reservoirs are found in Permian sandstones trapped by impervious salt domes. The natural gas derived from underlying Coal Measures, migrated upwards through the Permian sands.
The Triassic is so called from its threefold division in Germany, but in Britain the middle division (Muschelkalk) is missing. Here arid conditions continued throughout the period, beginning with the Bunter Sandstone (Buntsandstein) and Pebble Beds, now part of the Sherwood Sandstone Group. Braided rivers and flash flooding transported coarse sands and gravels from the Variscan highlands to produce cross stratified sandstones, pebble beds and conglomerates that are characteristic of the Lower Triassic strata. Examples of the pebble beds can be seen in Staffordshire on Cannock Chase, High Shutt near Alton Towers and in Park Hall Country Park. The Budleigh Salterton Pebble beds in South Devon also show evidence of braided rivers and wind faceted 3 sided driekanter pebbles on desert surfaces. The overlying Keuper Marl now called the Mercia Mudstones comprise red-brown clays up to 1000 metres thick in the English Midlands and they contain considerable deposits of halite and gypsum. These were laid down in shallow playa lakes as evaporites. Note that rock salt was extensively worked around Northwich and Droitwich and gypsum was mined in the Newark area. At the end of the Triassic, the Rhaetic marine transgression heralded the onset of the Jurassic period as the shallow marginal waters of the advancing Tethys Ocean covered the desert plains and playa lakes. The Rhaetic bone bed represents a condensed deposit of fish teeth, scales, and bones, coprolites from aquatic reptiles and early dinosaur bones that may be the result of mass mortality due to salinity changes or simply the sweepings of  the advancing seas.
Commercially the Sherwood Sandstone Group is a major aquifer providing water for both domestic and industrial users throughout the Midlands. Where it is sandwiched between the underlying impervious Upper Carboniferous strata and the overlying Mercia mudstones, the porous and permeable Sherwood sandstone ( up to 600 metres thick) can yield up to 125 litres per second from a vast underground reservoir.  Coarse grained sandstone is both highly permeable and porous since there are spaces between the grains which allow water to pass through and also be stored.
"To see the world in a grain of sand.........(and) hold infinity in the palm of your hand." William Blake
John Downes
Here are some views looking towards Gilman point.
Here are some views most of which are attributed to Myles who did a great job with his camera.
We gathered at Stepaside Ironworks [SN141074] at 10.30 am on the last day of flaming June  with shorts and sunhats at the ready. Our first task was to identify the building stones used in this magnificent Victorian edifice. The orange brown colour of the stonework reflected the iron rich Coal Measure sandstone and some blocks glinted in the sunlight where quartz crystals had developed within joints in the sandstone; muscovite mica flakes could also be recognised on some surfaces. When we reached the wall of the casting house there was ample evidence of iron smelting in remnants of slag where we discovered fragments of ironstone, anthracite and limestone.
This site of industrial archaeology reflects the importance of economic geology in the 19th century in South Pembrokeshire. All the materials for iron making were available locally; anthracite, iron ore and limestone, and the Saundersfoot Railway had already been extended to Stepaside when the ironworks opened in 1849. However, supplies of bedded iron ore from the coalfield were limited and as economic conditions declined, the ironworks was forced to close in 1874. Today the ruined buildings have been restored and developed as a tourist attraction.
The stone casting house with its three arched doorways and circular openings stands as a monument to solid Victorian engineering. The two blast furnaces originally stood in front of the furnace wall, one to each pair of arches. The furnaces were loaded by overhead bridges leading from the upper platform where the raw materials were assembled. Before the furnaces were charged, the anthracite was converted into coke in the coke ovens and the iron ore was roasted in the calcining kilns. Compressed air to aid combustion was provided by the blowing house, which is the tall building to the right of the furnace wall.

After exploring the ironworks, we climbed up the pathway to the remains of Grove colliery. This mine was opened in 1853 to supply anthracite from the Kilgetty Vein. A branch line from the Saundersfoot railway linked the colliery to the ironworks. Today you can still see the ruins of the winding engine house that lifted the cages up and down the shaft, and the Cornish pumping house that contained a 274 hp beam engine that raised water from the mine

From Stepaside we drove across to Pendine for lunch; this seaside resort  was crowded with holiday makers and those attending the motor rally on Pendine Sands. However, we managed to leave the crowds behind as we traversed westwards from Dolwen Point [SN234079] to examine the Carboniferous limestone succession that forms the northern boundary of the Pembrokeshire and South Wales coalfields. The base of the limestone is seen at Dolwen Point and the strata dips south west for about 1 km to Gilman Point where the top of the succession is exposed. At Dolwen Point the Avon Group (Lower Limestone shales) is composed of well bedded and jointed grey limestones in the centre of which is a thick bed of calcareous mudstone. The mud has undergone soft sediment deformation induced by gravity on a gently inclined depositional slope. There are slump folds, convolute beds, pillow beds and fractured blocks that have sunk into the mud; the mudstones are sandwiched between more competent limestone beds.
The Abercriban Oolite overlies the Avon Group near the cliff steps just west of Dolwen Point. This is a massively bedded bioclastic limestone where we examined fallen blocks containing shell beds and lenses with productid brachiopods (Dictyoclostus), single corals (Zaphrentis) and colonial corals (Lithostrotion junceum).  We then made our way across the sands to Gilman Point where the massively bedded Dowlais Limestone is exposed. On the western side of the headland the limestone is overlain by cemented periglacial head deposits up to 10 metres thick containing large clasts of quartzite and limestone. This material appears to have slipped down the side of a preglacial valley having been lubricated by surface meltwater as the frozen ground began to thaw at the end of the Devensian glaciation in a process known as solifluxion. On returning along the beach we discovered some remains of a submerged forest. The submergence relates to the Flandrian marine transgression at the end of Pleistocene times.
John Downes
Geology / Re: Geology Group Diary (32)
June 14, 2018, 07:34:48 pm
The last images for the Coal measures.
Geology / Re: Geology Group Diary (32)
June 14, 2018, 07:30:45 pm
Here are the next 6 images for the Upper Carboniferous.
Geology / Geology Group Diary (32)
June 14, 2018, 07:27:21 pm
The Geology Group met at 10.30am on Wednesday 13 September 2018 at Merlin's Bridge Village Hall. The topic for the meeting was:
By the beginning of the Namurian (325-315 Ma) much of Northern England had became a region of swampy delta plains and shallow lakes, with large rivers draining south off the northern ORS continent. The muds, sands  and coal seams were laid down in rhythmic cycles of sedimentation (cyclothems) as sea level fluctuated throughout the Upper Carboniferous. The Millstone Grit so characteristic of the Pennines, represents major deltaic development during the Namurian. And by Westphalian times (315-300 Ma) delta top swamp forests flourished intersected by numerous distributary rivers. The typical Coal Measure cyclothem of shale, sandstone and coal seam represents the changing environmental conditions ranging from the marine submergence of the delta plain, through the development of thick deltaic muds and sands to the growth of swamp forest on the delta top.
South of St Georges Land, the Rheic Ocean was closing with the resultant formation of the Variscan Mountain ranges that stretched from the Appalachians through the Pyrenees and Atlas Mts to the Urals in Russia. This Variscan Front can be seen in SW England where the Culm Measures (sandstones, mudstones and thin coals) were deposited in a subsiding basin (Devon syncline). Variscan folding and thrust faulting at the end of the Carboniferous can be seen particularly well in the South Wales and Pembrokeshire coalfields. The coastal sections at Broad Haven and Saundersfoot show intense thrusting and folding within the Coal Measures and the chevron folds at Millook Haven in North Devon are classic Variscan features. The intrusion of the granite batholith in Cornwall and South Devon (Dartmoor, Bodmin Moor,etc) is associated with the Variscan earth movement as pressure built up from the south.
In late Carboniferous times sheets of molten magma were injected into the rocks of the northern Pennines. 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.
In the Midland Valley of Scotland there was considerable vulcanicity throughout Carboniferous times. The Clyde Plateau basalts are up to a 1000 metres thick and Arthur's seat in Edinburgh represents an extinct Carboniferous volcano. Salisbury Crags is a massive sill where James Hutton proposed his ideas on the origin of igneous rocks in the late 18th C. 
THE PENNINES  In the Peak District and  the Yorkshire Pennines the Millstone Grit forms dramatic escarpments where it has been removed by erosion from above the underlying shales and limestone as for example on the Roaches, on Stannage Edge, on Kinderscout or on Ilkley Moor. The term 'grit' is commonly used but it is actually a coarse sandstone that was employed extensively in the 19th C to build the 'dark satanic mills' and the back to back terraced houses of industrial Yorkshire. The dark exterior colour of Millstone Grit is largely due to industrial pollution since in fresh specimens the rock is a light colour due to the abundance of quartz. The gritstone bands alternate with shales and thin coal seams in a series of cyclothems. Note that cross stratification is often seen in the gritstones, evidence of their deltaic origin.
The Millstone Grit passes conformably into the Coal Measures which outcrop on the flanks of the Pennines. On the Yorkshire coalfield the strata dips gently eastwards so that the exposed coal measures are eventually concealed beneath a cover of younger Permo-Triassic rocks. Here the cyclothems are fully developed with some seams reaching up to 3 metres in thickness. During Victorian times the exposed field fuelled the industrial revolution, but as these seams were worked out deeper pits were sunk down to 800 metres in the concealed field. Kellingley Colliery in North Yorkshire was a deep mechanised pit developed in the 1960s to feed the Ferrybridge Power station, but the decline of coal fired power stations and the uneconomic cost of deep mining, resulted in the mine was closure in 2015...it was the last deep coalmine in the UK.
Here are some images of West Angle Bay.