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Messages - johnd

Geology / Re: Geology Group diary (37)
December 15, 2018, 05:01:28 pm
More images of the Quaternary.
Geology / Geology Group diary (37)
December 15, 2018, 04:52:40 pm
The geology Group met at Merlin's Bridge village hall at 10.30 am on Wednesday 12 December 2018.
The topic for this meeting wasBRITISH QUATERNARY GEOLOGY.
Since the end of the Tertiary Period around 2.5 Ma, the climate of Britain has fluctuated between cold glacial episodes and warmer temperate episodes. These climatic cycles are thought to be controlled by predictable changes in solar radiation (Milankovitch cycles). The Quaternary is divided into the Pleistocene epoch that extends up to 10,000 years ago, and the present day Holocene which is probably the latest interglacial phase. It is only during the last 300,000 years that we have direct evidence of glaciation in Britain where three major glacial episodes and interglacials can be recognised.
The earliest deposits of the Pleistocene are those of the Red Crag of East Anglia. These are shallow water marine sediments containing molluscs that indicate a cool temperate climate. They were laid down as sands and gravels on the margins of the North Sea basin. Today the iron stained sands of the Red Crag are well exposed in the cliffs of Walton on Naze. There are several more younger crag deposits overlying the Red Crag; these include the Norwich Crag and the Weybourne Crag. The latter was the last marine deposit in Britain and contains a high proportion of  molluscan arctic fauna; in fact there is a gradual decrease in the percentage of warm water shells throughout the Lower Pleistocene indicating a cooling of the climate. The succeeding Cromer Forest Bed  comprise two freshwater beds and  an estuarine bed that were laid down around 350,000 years ago. Large numbers of mammalian remains have been found in these beds including deer, sabre toothed tiger, hyaena, rhino and mammoth. Many bones appear to have been washed down by rivers along with plant remains that indicate a temperate climate.
However, towards the top of the sequence ice wedges and periglacial soils suggest the onset of the beginning of the Anglian glaciation (300-250,000 years ago) This is the earliest ice advance in Britain that  has left depositional evidence. The ice extended as far south as the Bristol Channel and the Thames valley and the course of the Thames was diverted by the advancing ice front southwards to its present position. In East Anglia the North Sea Drift (or Cromer Till) is formed of glacial till (also known as boulder clay) deposited as the ice sheets finally retreated. The till contains erratics from Scandinavia including a distinctive dark blue igneous rock called larvikite. However, the largest erratics are locally derived chalk rafts up to 100 metres in length; after subglacial plucking from the bed of what is now the North Sea, the blocks were moved by the ice sheet and then deposited when the ice melted. Deformation structures known as Contorted Drift can be seen in the coastal exposures where glacial sediments have been deformed by ice pressure and movement. The Cromer Ridge (14 kms long) is a conspicuous landform representing a terminal moraine 90 metres high that was formed along the margins of  successive ice sheets moving southwards towards the north Norfolk coast. The coast around Cromer displays sections through the moraine showing its internal structure.
The Hoxnian interglacial represents a return to temperate conditions after the Anglian glaciation. A rich flora and fauna include oak and alder plus mammalian remains but also skull fragments of early Homo sapiens. Sea level was 20-30 metres above the present day level as melting ice increased the volume of sea water.  The next glacial phase is known as the Wolstonian (200-130 years ago) when the ice sheets extended south across the Midlands. The vast proglacial Lake Harrison was formed when meltwater was ponded up between the ice front and the Cotswold escarpment. The succeeding Ipswichian interglacial was characterised by temperate conditions with an abundance of oak woodland as shown by. pollen analysis. Numerous mammals roamed the lowlands including elephant, bison, deer and rhinos and these were hunted by early humans. Eventually, the climate became cooler and the Devensian glaciation began around 115,000 years ago. The ice front did not reach as far south as in previous glaciations but ice lobes extended over the lowlands as in the Vale of York, the Cheshire plain and the Irish Sea. Terminal moraines like those at York and Escrick and other depositional features (drumlins, eskers and kames) are well preserved since they are less weathered than those of earlier glaciations. During recession of the ice front meltwater was again ponded up between the ice and high ground. The eastward drainage of the Vale of Pickering was blocked by glacial debris and the Derwent diverted southwards to the Ouse through the Kirkham Abbey overflow channel. A similar drainage diversion took place when the Severn which formerly flowed north, was blocked by ice and Lake Lapworth created. The waters later overflowed through the Ironbridge Gorge to create a southerly route for the river.

Some 10,000 years ago rising temperature caused the ice to melt and sea level rose by up to 100 metres. This was known as the Flandrian transgression when the English Channel was created along with numerous drowned estuaries (rias) such as the Milford haven, the Fal estuary and Poole harbour. The raised beaches in western Scotland for example, are evidence of higher sea levels at this time, but they also demonstrate the effect of isostatic uplift because when the ice melts, the land rises as the weight of ice is removed.
Landforms produced by Glaciation.
During the Pleistocene glacial phases highland Britain was covered by ice sheets at least 1000  metres thick. Ice moved outwards from the Scottish highlands, the Lake District and the Welsh mountains and valley glaciers moved down all the major valleys. The preglacial valleys were scoured and deepened by moving ice into U shaped troughs. Tributary streams may be left in hanging valleys above the main valley creating spectacular waterfalls. The glaciers were nourished in great hollows known as corries or cwms that were excavated on the mountain sides. The corries often became separated by knife edged ridges called arĂȘtes. Bare rock outcrops on the valley floor were smoothed on the upflow side and plucked on the leeward side to form distinctive roche moutonĂ©es. Long ribbon lakes occupy some over deepened valleys, as for example in the Lake District.
Glaciated  lowland areas are often covered in drift materials such as sands, gravel and boulder clay ( glacial till). Boulders that have been transported by ice from distant sources are called erratics and these are useful in determining the direction of ice flow. The Austwick gritstone erratics in North Yorkshire weigh several tonnes and have been moved a few kilometres on to the limestone pavement. Meltwater flowing under a stagnant ice sheet often forms sinuous ridges of sand and gravel known as eskers. The Blakeney esker in Norfolk is a good example of this type of feature. Where boulder clay is deposited beneath an ice sheet in a valley it may be moulded into elliptical shaped mounds called drumlins that are aligned in the direction of ice flow. Moraines are accumulations of ill-sorted glacial debris that have been transported by ice and then deposited when the ice melts; terminal moraines form at the ice front marking the maximum advance of the ice; lateral moraines form along the side of a glacier.
John Downes
Geology / A Big Thank You
December 13, 2018, 03:02:50 pm
Dear Geology Group members,

This is a note to thank you all for your cards, good wishes and kind comments now that I am leaving Pembrokeshire. It has been a pleasure and a privilege to be able to talk about geology to such an enthusiastic group. Thank you for your support and interest over the years. I wish you all the very best for the future.

Kind Regards,

Geology / Re: Geology Group Diary (36)
November 14, 2018, 09:43:52 pm
Finally 6 more images.
Geology / Re: Geology Group Diary (36)
November 14, 2018, 09:38:53 pm
More images of Tertiary rocks.
Geology / Geology Group Diary (36)
November 14, 2018, 09:34:30 pm
The Geology group met at 10.30am at Merlin's Bridge village hall on Wednesday 14 November 2018. The monthly topic was TERTIARY GEOLOGY IN BRITAIN.

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 and Ardnamurchan volcanic centres are 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 caldera 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!
Tertiary sediments in Southern England
Whilst volcanoes were erupting in the north west of Britain, sedimentation was taking place in southern England. During the Palaeocene and Eocene epochs the London and Hampshire basins were subject to alternate marine transgression and regression.
London Basin. 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.
In the Hampshire Basin (which includes the IOW) several cycles of sedimentation can be distinguished in the Hampshire Basin which roughly correspond to those in the London basin.  One of the best places to examine the Palaeogene succession is Alum Bay on the NW side of the IOW. Here the cliffs are formed mainly of multicoloured sands and clays (Bracklesham Beds) that are stained red, brown and green by iron compounds derived from pyrite (FeS2) which is oxidised above high tide level. There are also brown coloured beds of lignite and several plant beds providing evidence of a subtropical climate. The overlying Barton Beds are best examined on the Hampshire coast at Barton-on-Sea, east of Christchurch. Here the Barton Clay yields numerous bivalves (eg.Cardita) and gastropods (eg.Turritella; Volutospina) and sharks'teeth.
Much of the eastern part of East Anglia contains marine shelly sands known as Crag deposits. The Coralline Crag is of Pliocene age has an abundance of fossil bivalves, gastropods and brachiopods. The overlying Red Crag, Norwich Crag and Weybourne Crag form the lower part of the Pleistocene sequence. The molluscan fauna indicate a gradual climatic cooling from subtropical species in the Coralline Crag to boreal species in the Weybourne Crag, heralding the onset of the Pleistocene Ice Age.
The Alpine Orogeny.
The Alpine fold mountains were formed when Africa collided with Eurasia and the Tethys ocean crust was subducted northwards beneath the Eurasian plate. The Pyrenees, Alps, Apennines, Carpathians and Caucasus ranges were all formed by the Alpine orogeny, although some like the Pyrenees originated as Variscan structures. These earth movements occurred throughout the Tertiary but reached a peak during the Miocene when the Mediterranean was formed as a large saline basin on the site of the vanished Tethys Ocean. In southern Britain the ripple effect of Alpine crustal movements produced structures such as the Wealden anticline, the London and Hampshire synclinal basins and the Purbeck and IOW monocline.

Geology / Re: Geology Group Diary (35)
October 11, 2018, 08:29:16 pm
6 more images of Cretaceous geology.
Geology / Geology Group Diary (35)
October 11, 2018, 08:24:45 pm
The Geology Group met at 10.30am on Wednesday 10 October 2018 at Merlin's Bridge Village hall. 20 members present. The topic this month was...THE CRETACEOUS ROCKS OF BRITAIN.
By the beginning of the Cretaceous period, Pangaea was breaking up accompanied by increased plate tectonic activity. The proto Pacific, Indian and Atlantic oceans were formed at this time as spreading mid oceanic ridges produced large volumes of basaltic lavas causing a eustatic rise in sea level. The Tethys Ocean began to close in late Cretaceous times as the Arabian/Indian plates drifted northwards towards Eurasia.
The Cretaceous (145 to 65 Ma) is the longest geological period since the end of the Precambrian. The Weald of south east England provides one of the best areas for the study of Cretaceous strata.. 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 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. The first remains of Iguanodon were discovered in 1822 by Mary Mantel in the Weald of Sussex. Ironstone nodules occur within clay bands providing a 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 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 when bricks were in demand for the London market.
The Lower Greensand 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 orange or brown in colour due to oxidation, but the name came from greensand containing glauconite (hydrated potassium iron silicate) that outcrops on the Dorset coast. The two most prominent formations within the Lower Greensand are the Hythe beds (buff sandstones with chert layers) and the overlying Folkestone beds (orange/brown poorly cemented sandstones). The latter often show cross stratification; evidence of deposition in shallow seas. They also contain irregular contorted beds of ironstone (carstone) that were precipitated by ferruginous waters percolating through the sandstone after it was lithified. The Folkestone beds are quarried to provide soft building sand.
The Lower Greensand was deposited under shallow marine conditions as the Wealden Lake was invaded by the sea around 115 million years ago. Later as the sea deepened the Gault Clay was laid down. This 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 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.
Mass Extinction at the end of the Cretaceous
This event is known as the K-T extinction or the K-Pg extinction (Cretaceous-Tertiary or Cretaceous- Palaeogene). It occurred 66 million years ago when an asteroid some 10-15 kilometres in diameter impacted the Earth creating the Chicxulub crater in the Yucatan peninsula in the Gulf of Mexico. The boundary marking the extinction is formed of a thin layer of sediment that is rich in iridium which is abundant in asteroids and meteorites. The sediment represents the dust and shattered rock fragments produced by the impact. Shocked quartz which is produced by intense pressure, is also present in the sediment. Luis Alvarez, an Italian physicist, first proposed the impact hypothesis in the 1980s when he identified the iridium rich clay boundary layer near the ancient Umbrian town of Gubbio. Later the Chicxulub crater was discovered in the 1990s and it provided strong evidence in support of Alvarez's research.
However, it is likely that other events contributed to the mass extinction. In the Deccan plateau in India basalt lavas were erupted at the end of Cretaceous times. These would have produced vast amounts of CO2 and SO2 and contributed to global warming with acid rain killing off the vegetation. Whilst 75% of all species became extinct, the dinosaurs are often seen as the chief victims of catastrophic events, yet many invertebrates such as the ammonites had been in decline throughout the Cretaceous. Other creatures including bony fish and placental mammals developed during the Cretaceous, survived the K-T impact and then expanded in the Palaeogene.
John Downes
Geology / Re: Geology Group Diary (34)
September 13, 2018, 05:27:01 pm
Finally from the Dorset Coast.
Geology / Re: Geology Group Diary (34)
September 13, 2018, 05:23:23 pm
Here are 6 more images of the Jurassic strata.