Solid Geology
The Vale of York is a low-lying alluvial basin stretching for over 50 km from the vicinity of Northallerton in the north to the Doncaster-Gainsborough area in the south. It is bounded to the east by the North York Moors, Hambleton Hills and Howardian Hills, which consist mainly of Jurassic sandstones and limestones, and by the Yorkshire Wolds, largely comprising Cretaceous chalk. On its western side, the vale is flanked by low foothills of Permian dolomitic limestones, beyond which are the Carboniferous uplands of the Pennines. Triassic sandstones and ‘marls’ form bedrock beneath the vale, but these rocks are largely concealed by Quaternary sediments, principally of glacial, lacustrine, aeolian and riverine origin. Most of these sediments were deposited during the last cold Stage (the Devensian) and the succeeding post-glacial Stage (the Holocene).
Pleistocene Environments
During the last glacial maximum between c. 26,000-13,000 years before present (the Dimlington Stadial), glacier ice fed from snow accumulation centres in the Lake District, Cheviots and further north, entered the Vale of York and was augmented by ice flowing down Swaledale, Wensleydale, Nidderdale and Wharfedale from local Dales accumulation centres. Glacial and meltwater processes within and beneath the icesheet led to the deposition of extensive deposits of till and fluvioglacial sands and gravels. Ice flowing down the east coast blocked the Humber Estuary, resulting in drainage impediment and the creation of proglacial Lake Humber. This lake rose initially to a short-lived high level of about 30 m OD (Ordnance Datum), with littoral sands and gravels deposited around the margins.
Its level then dropped transiently to below minus 4 m OD, possibly draining away entirely, before rising again to a more prolonged low-level of approximately 7.5 m OD. The sediments formed in Lake Humber, mainly during the later, low-level phase, consist largely of laminated clays with marginal sands. These deposits, together with upper sands of post-lacustrine fluvial origin, are for historical reasons known collectively as the 25-Foot Drift.
The position of this ice margin during this prolonged lower phase is marked by the York and Escrick moraines. A bone fragment found within the littoral sands and gravels near Brantingham, near South Cave, yielded a radiocarbon age of nearly 22,000 years BP, and provides an estimated age for the high-level lacustrine phase. Climatic amelioration around 13000 years BP (associated with the Windermere Interstadial) led to the retreat and wastage of the Vale of York glacier. A minimum age for the deposition of the silts and clays of Lake Humber is provided by the radiocarbon date of approximately 11,000 years BP gained from the organic part of a soil developed on these sediments at West Moor, north-east of Doncaster. The lake eventually disappeared through silting up rather than by drainage and the upper sands of the 25-Foot Drift, which form low discontinuous ridges and mounds, are interpreted as levées deposited by rivers flowing across this newly exposed land surface.
Between 11,000 and 10,000 years BP (the Loch Lomond Stadial), colder climatic conditions returned to the Vale of York, creating a tundra environment. Within this sparsely vegetated landscape, the finer grained fluvioglacial deposits and the upper sands of the 25-Foot Drift were subject to processes of wind erosion, creating extensive sheets of blown sand and dune landforms to the east and north of York.
