A re-interpretation of the physiographic evolution of the southern end of the Vale of York from the mid-Pleistocene to Early Holocene

The recognition and mapping of planar terraces on the York Moraine led to the belief that these were shorelines of the Late Devensian proglacial Lake Humber and the hypothesis that the progressive demise of the lake was recognisable from stillstands. To test this, landform mapping was initiated across the Vale of York and the flanks of the Wolds between Pocklington and Hessle to identify and record planar land surfaces, which had distinct topographic boundaries resulting from erosional and depositional processes. The results of this study confirmed the early shoreline mapping and identified strandlines, at lower elevations, down to a terminal lake level of 5.0 m OD. Erosional and deposition effects associated with both stands and retreat stages of Lake Humber have deposited a sand mantle up to 2.0 m thick on the southern face of the York moraine from transgressive and regressive shorelines. In addition, two sets of alluvial fans, originating from dry valleys in the Wolds from frost-fractured Chalk formation were recognised. The older set were terraced by shorelines of Lake Humber, in contrast to the younger set, which clearly post-dated Lake Humber. Corroborative evidence for the existence of the shorelines has been provided by photography and LIDAR imagery. An additional objective was to establish a chronology for key mapped landforms based on luminescence dating of sand samples from shoreline deposits and younger fluvial events. To achieve this 18 sand samples were collected and dated. The main conclusions of the research are that the older periglacial alluvial fans are from an earlier cold glacial period (possibly MIS 8) and that the younger Late Devensian (MIS 2) glaciation retreated north of the York Moraine about 17 ka BP prior to the main phase of impounding Lake Humber. The dating of this event conforms with OSL dating of c. 15.9 ka and c. 15.2 ka obtained from sand samples on the York Moraine. The existing two stage model of proglacial Lake Humber has been revised and mapped shorelines, of this lake, now define an 8-stage regressive decline model for Lake Humber, from a high level Stage 1 at 42 m OD down to a low level (partly fluvial) Stage 8 at 5.0 m OD. Mapping has also revealed that the decline sequence appears to have been punctuated by short-lived, modest rises in lake level from oscillations of the blocking North Sea Ice Lobe. It has been established that the southward slope of the Vale of York is testimony to Holocene flooding and not to isostasy. The landform mapping also indicates that the gravels forming the Crockey Hill ‘esker’ probably originated as a fan delta from a drainage gap in the York Moraine, south of York, and that terraces in the Aire and Calder valleys are coeval with stands of Lake Humber.