Alluvium, Quaternary, Bristol and Gloucester region

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Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)

Alluvium[edit]

The south face of the Mendips seen across the alluvial flats of the Somerset Levels (P206222).

Alluvium is more extensive than any other Quaternary deposit. The Somerset Levels, (P206222) between the Quantock and Mendip hills, form the second largest fenland in England. Other great spreads occur on either side of the River Severn as far north as the Severn Bridge at Aust and beyond.

The alluvial deposits have filled, to mean high-water mark (about 6 m above OD), an extensive buried valley system which was graded to the low sea level prevailing at the end of the Devensian. The thickest succession encountered so far lies to the south-west of Brent Knoll, in the Somerset Levels, where it has been proved to 30 m below OD. The site lies some 7 km inland and it is probable that greater thicknesses are present seaward. The base of the succession is usually marked by a peat layer associated with tree stumps in situ, that are exposed from time to time at low tides along the coast. Radiocarbon measurements show that the age of the basal peat increases with depth below OD, and in the Somerset Levels a maximum of around 8500 years has been recorded at about 20 m below OD. As the sea level rose, the forest died and gave way to swamp, now represented by the basal peat, before being finally overwhelmed by the sea. In the thickest alluvial successions, the basal peat is usually followed by sands, which are succeeded by intertidal, laminated, grey silty clay and fine sand and, finally, up to about OD, by grey clay or peaty clay, representing salt marsh.

Periodic re-invasions by land vegetation along the coastline are represented by thin peat layers around the margins of the levels. About 5500 years ago, however, the widespread clay surface that now lies at about OD was briefly colonised by forests of oak and pine, which were then killed by poor drainage with the establishment of peat mires. Subsequent encroachment of this so-called ‘OD peat’ by marine clays never extended beyond about 10 km inland from the present coast, leaving raised bogs farther inland. At Shapwick and Meare, these persist to the present day. The inland peat is up to 5 m in thickness and has been exploited, probably since Roman times.

In the remainder of the region the rivers and all but the smallest streams have deposited spreads of loamy alluvial silt.