Younger Dryas glacial landsystems in North West Scotland: an assessment of modern analogues and palaeoclimatic implications

By explicitly linking process to form, landsystem models developed in modern environments provide powerful tools for the reconstruction of past environments from the sedimentological and geomorphological record. Some landform characteristics, however, may originate by more than one process (equifinality), so although a particular depositional model may "fit" the data, it is not necessarily the correct one. Here, we critically examine the use of modern analogues to interpret glacial landforms, focusing on "hummocky moraine", a distinctive landform association consisting of numerous sharp-crested moraine mounds and ridge fragments, which occurs on many valley floors in upland Britain. Detailed studies of examples of Younger Dryas age in the North West Highlands of Scotland show that they represent ice-contact fans which have been affected to varying degrees by ice push and subglacial deformation, and that they formed at active, oscillating ice margins. The sedimentological data are inconsistent with an origin by englacial thrusting, as has been proposed for similar moraines elsewhere in upland Britain on the basis of morphological criteria. Comparison with modern glacial landsystems shows that, although aspects of "hummocky moraines" in the study area have close modern counterparts, none provides an exact analogue in all respects. This may reflect the distinctive climate of western Scotland during the Younger Dryas. Estimates of mean July temperatures in the Younger Dryas in Scotland, based on chironomid assemblages, are c. 8 C at sea-level. When combined with reconstructed equilibrium line altitudes for contemporaneous glaciers, these estimates indicate mean annual precipitation totals in the range of 2358 +/- 337 mm yr(-1) at sea-level in NW Scotland, c. 26% higher than present-day values. The implied heavy snowfall suggests that glaciers in western Scotland during the Younger Dryas were of high activity type, unlike modern glaciers in Svalbard, for example. High glacier velocities and high winter precipitation would suppress permafrost development, despite low mean annual temperatures. The climatic environment also favoured rapid oscillations of glacier margins, during both glacier build-up and retreat phases. Climatic factors, combined with the availability of large quantities of subglacial debris, led to the development of distinctive glacial landsystems, which may have no direct modern analogue. (c) 2006 Elsevier Ltd. All rights reserved.