A striated, far travelled clast of rhyolitic tuff from Thames river deposits at Ardleigh, Essex, England : evidence for early Middle Pleistocene glaciation in the Thames catchment

摘要

This paper reports the discovery of an in-situ striated, far-travelled, oversized clast in the Ardleigh Gravels of the Kesgrave Sands and Gravels ofudthe River Thames at Ardleigh, east of Colchester in Essex, eastern England. The morphology, petrography and geochemistry of the clast, and theudsedimentology of the host deposit are described. The striations are interpreted, on the basis of their sub-parallelism and the shape and subroundednessudof the clast, as glacial and the clast is provenanced to Ordovician rocks of the Llˆyn and Snowdonia regions of North Wales. On the basisudof clast frequency within the Colchester Formation gravels of the Kesgrave Sands and Gravels it is inferred that glaciers reached the Cotswold regionudof the Thames catchment. Floe-ice transport during spring flood is invoked for movement from the glaciated region to eastern England. The paperuddiscusses the possible age of the glaciation and recognises that it is difficult to be more precise than a cold stage in the early Middle Pleistoceneud(MIS 18, 16 or 14). Attention is drawn to the possibility of glaciation associated with a diamicton in the region of the Cotswold Hills known as theudBruern Till, but stresses the need for new work on this deposit.