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Abstract

The series of ridges, mounds and basins around Carstairs have been variously interpreted as recessional moraines, kames, sub- or en-glacial eskers or as landforms developed in response to supraglacial outwash fan sediments deposited on stagnant ice. This paper argues that the ridges are eskers. They formed in an interlobate sediment sink between the uncoupling margins of Highland and Southern Upland ice during Devensian deglaciation. This conclusion is based on geomorphological mapping, lithofacies and palaeocurrent analysis, and borehole interpretation. An extensive lake system developed between the two ice margins and was fed by a major sub-glacial conduit flowing towards the NE. This conduit initially exited into the lake subaqueously to form a prominent single esker ridge across the lake floor. On downwasting, the feeding conduit emerged onto the surface of the ice to form a complex supraglacial outwash sandur. During periods of catastrophic flow, large channels cut through the sandur surface into underlying ice. These channels filled with coarse gravel which, on abandonment and further downwasting, were topographically inverted to form a series of sub-parallel, slightly sinuous ridges giving the appearance of braiding. During lower flow extensive finer-grained supraglacial sandur sedimentation took place on the periphery of these ridges and this passed down-current across the ice margin into fan-deltas feeding into the expanding ice-front lake.