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Geology Group diary (37)

Started by johnd, December 15, 2018, 04:52:40 pm

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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


More images of the Quaternary.