Scottish 'hummocky moraine': its implications for the deglaciation of the North West Highlands during the Younger Dryas or Loch Lomond Stadial

Abstract

A small ice cap grew and decayed in the Scottish Highlands during the Devensian Lateglacial Loch Lomond Stage (C. 11 000 to 10 000 years B. P. ). Its extent has been largely determined by the distribution of Scottish "hummocky moraine" which has been interpreted in the past as stagnation terrain. This stagnation was considered merely to reflect the ice cap's rapid decay in response to the extremely rapid climatic warming at the close of the Loch Lomond Stadial with the evidence for this rapid climatic amelioration being drawn from the coleopteran record.

The analysis of 10 803 air photographs of "hummocky moraine", within the North West Highlands, has not revealed a disordered stagnation terrain, but a clear pattern of ridges which resemble those found at modern ice margins.

Detailed field investigation has shown that these ridges can be interpreted as suites of push moraines, dump moraines, flutes and outwash fans. Such landform suites are typical of many glacier margins that are actively decaying today. On a meso-scale these landforms have a spatial organisation similar to that found at modem ice margins: at a macro-scale they can be mapped over large areas to form a well integrated pattern. I suggest, therefore, that this pattern of drift ridges reflects the active decay of the Loch Lomond Stadial ice cap within the North West Highlands. On the basis of this interpretation the extent, surface morphology and pattern of decay of the Loch Lomond Stadial ice cap within the North West Highlands has been examined. It occupied an area of 4 700 km2 and incorporated a total volume of 1 900 km3 of ice. The pattern of decay is strongly influenced by topography while lithology and ice cap dynamics were found to be important controls on the distribution of drift left by this ice cap.

Attention is drawn to the apparent conflict between the relatively prolonged period of decay indicated by the geomorphological record and the rapid climatic amelioration indicated by the palaeo-biological record at the close of the Loch Lomond Stadial. This is explained in terms of the glacier-climate interaction both damping and delaying the response of the glacial system to climatic change.