Quaternary - the last 2-3 million years

The sub-divisions of the Quaternary are now based on the information contained in sediments, which contain the skeletons of micro-organisms obtained from sediments drilled from the seabed, where the succession of deposits is complete. The skeletons are made of calcium carbonate, which contains oxygen. The ratio of the heavier isotope of oxygen to the lighter reflects temperature so that it has been possible to calculate the number of cold and warm stages during the last 2 million years (Marine Isotope Stages). Problems arise when attempts are made to correlate the seabed record with the incomplete land record. Glacial deposits are limited in extent and inter-glacial deposits are preserved in very limited areas, added to which a great deal of erosion took place after such materials were deposited, so the record is very fragmentary. Recent work on evidence from ice cores taken from the Greenland Ice Cap have contributed significantly to the reconstruction of climate change in the later part of the Quaternary.

In South Gloucestershire the evidence for events during the Quaternary is very limited and most of the story is built up by inference from adjacent areas. E.g. in North Somerset glacial deposits in the area of Kenn and on the Failand Ridge and in Nightingale valley have been dated as pre Marine Isotope Stage 15 (MIS 15). Which means that they are older than 700,000.There is evidence that glaciers entered the Bristol Region some 450 thousand years ago (Marine Isotope Stage 12) known as the Anglian Glaciation. Ice damned the outlets of rivers, which may account for the anomalous drainage patterns, but little can be proved. We do know that the Warwickshire Avon changed from a northeast flowing river to a southwesterly one, joining the Severn and so increasing the size of its catchment.

Some 280 thousand years ago ice formed glaciers in the Midlands and South Wales, leaving South Gloucestershire ice free, but subject to a very cold climate which caused shattering of bedrock and probably a great deal of erosion.

More oscillations of climate took place before the last interglacial period, Marine Isotope Stage 5e, known as the Ipswichian, when sea level was high causing the Severn Estuary to be much wider than it is today reaching the former cliff line which extends from Easter Compton in a north-easterly direction.

During the last Cold Stage, Marine Isotope 2, known as the Devensian, the cold climate caused rock shattering, land slipping and a drainage pattern developed on the frozen ground in periods of summer melting. Loess, wind blown silt, has also been recorded.

In the region, (e.g. on the Cotswolds) the major event, which affected the River Severn, was the glacial diversion in Shropshire, which caused the Ironbridge Gorge and consequently enlarged the catchment of the River Severn to include all of mid-Wales. The main glaciation ended about 14 thousand years ago, However, there was another cold period which lasted some 500 years around 10,000 years ago. This was caused by the sudden cooling of the North Atlantic due to the North American Ice sheets collapsing and discharging vast amounts of icy water into the ocean. This cooling is reflected in the landscape by spreads of gravel emerging from some of the valleys, rock shattering and by the spread of wind-blown sand and silt which has become incorporated into the soils.

It is the events during the Quaternary which brought about the most recent geological processes. The last 10,000 years, known as the Holocene has seen very little geological activity in this area. Sea level rise caused by the melting of the Devensian ice sheets drowned the estuary. An ever-increasing supply of sediment, mostly due to human activity associated with agriculture, such as tree removal and ploughing has provided the material for the development of mud flats. Other significant landscape processes include soil development, peat development and the erosion associated with a gradual rise in sea level. The largest deposits formed in the Holocene are the peat and the mud flats along the River Severn.

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