Landforms and the geological evolution of the Vale of York during the Late Devensian

摘要

Preliminary landform mapping over the York terminal moraine showed that persistent terraces on the sides of the moraine are strandlines that must have formed during regression of pro-glacial Lake Humber. Of widespread significance is a conspicuous shoreline between 30 and 35 m above OD that lasted for sufficient time to develop both erosional and depositional lacustrine terraces. This surface is contemporaneous with the 100 Foot Strandline mapped on the western side of the Vale of York, on the Permian escarpment. Between the Escrick and York moraines there are two additional major strandlines,with associated littoral and lacustrine deposits, that effectively subdivide the fluvioglacial sediments of the Vale of York into three offlapping planar landforms, with lateral erosional contacts independent of lithology. Above the 100 Foot Strandline, mounds and ridges of till form prominent landmarks, such as Severs Howe near York. The North Sea ice could not have plugged the Humber mouth until the Vale of York glacier had formed the York Moraine, and must have remained long enough for a prolonged stillstand of Lake Humber at c. 33 m above OD. Subsequent stillstands, recorded on the southern face of the York Moraine at c. 20 m and c. 14 m above OD (lowering to c. 15 m and c. 10 m above OD at Escrick), mark stages in the unplugging of the Humber mouth from either ice or till. The mapping also suggests that the Crockey Hill 'esker' consists of erosional remnants, preserved on interfluves, of a fluvial gravel sheet.