Loch Lomond Stadial Plateau Icefields in the Lake District, Northwest England

Detailed geomorphological mapping has revealed evidence for the development of plateau icefields in the central fells of the English Lake District during the Loch Lomond (Younger Dryas) Stadial (c. 12.9-11.5 ka BP). The largest plateau icefield system, which covered an area of approximately 55 km2 (including outlet glaciers), was centred on High Raise. To the west, smaller plateau icefields developed on Grey Knotts/Brandreth, Dale Head and Kirk Fell, covering areas of 7 km2, 3 km2 and 1 km2 respectively. The geomorphological impact of these plateau icefields appears to have been minimal on the summits, where the survival of blockfields and other frost-weathered debris (mostly peat-covered) implies the existence of protective, cold-based ice (the Loch Lomond Stadial was the last major episode of periglacial activity to have affected upland Britain). As such, these represent the first reported occurrences of Loch Lomond Stadial ice masses which were not wet-based throughout. Cold-based conditions would have been promoted by a combination of thin, slow-moving ice plus the influence of low mean annual air temperatures on the summits. Ice-moulded bedrock at some plateau edges, however, document a transition to wet-based, erosive conditions. At these locations, steeper slopes would have resulted in increased strain heating within the ice. In many cases, prominent moraine systems were produced by outlet glaciers which descended into the surrounding valleys where their margins became sediment traps for supraglacial debris and inwash. In some valleys, ice-marginal moraines record successive positions of outlet glaciers which actively backwasted towards their plateau source. Given the virtual absence of periglacial trimlines within the area, reconstructed palaeo- ice margins constitute the single most important line of evidence in the identification of plateau icefields in the geomorphological record. The virtual absence of ice-marginal control points in the upper reaches of these plateau icefield systems means that their reconstructions are somewhat speculative. Ice thicknesses on the summits (40-50 m) are estimates based on the theoretical relationship between plateau icefield depth and summit breadth. A regional Loch Lomond Stadial firn line of 500 m OD is suggested for the central Lake District. This corresponds to a mean annual precipitation of 2,000-2,500 mm, assuming a mean July temperature of