Late Quaternary alluviation and valley floor development in the upper Axe valley, Mendip, southwest England

Abstract

Geomorphological, sedimentological and palaeoecological investigations of alluvial deposits in the upper Axe valley, southwest England, have identified two coarse grained alluvial units of late Quaternary age. The oldest gravel unit, the Wookey Station Gravel, is probably of Devensian age. On the basis of its topographic position, morphometry and sedimentary sequence it is interpreted as an alluvial fan, now dissected, deposited by a low-sinuosity gravel bedload stream. Terrestrial molluscs recovered from fossiliferous silts within this unit are typical of those found in many cold stage British Pleistocene deposits. The younger alluvial unit, the Wookey Village Gravel, dates from the early Holocene and was deposited sometime between 9500−8800 B.P. on the basis of pollen, molluscs and radiocarbon evidence. Lithofacies association within this unit (gravelly lateral accretion surfaces, asymmetrical palaeochannel fills) indicate deposition by a meandering gravel-bed river. Alluvial sediments in the upper part of the Axe valley record in some detail hydro-climatological related changes in stream run-off and rates of sediment supply that occurred during late Quaternary times in Mendip.