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Geological Society, London, Special Publications; 2000; v. 176; p. 321-336;
DOI: 10.1144/GSL.SP.2000.176.01.24
© 2000 Geological Society of London

Glaciotectonic Structures

Moraine-mound formation by englacial thrusting: the Younger Dryas moraines of Cwm Idwal, North Wales

David J. Graham1 & Nicholas G. Midgley2

1 Centre for Glaciology, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Credigion SY23 3DB, UK djg97{at}aber.ac.uk
2 School of Biological and Earth Sciences, Liverpool John Moores University, Byrom Street, Liverpool L3 3AF, UK

The Younger Dryas (c. 11-10 ka BP) moraine-mound complex (‘hummocky moraine’) in the historically important site of Cwm Idwal, North Wales, has previously been interpreted using periglacial, subglacial, ice-marginal and englacial models. In this paper the morphology and sedimentology of these landforms is described and the competing hypotheses tested against this evidence. It is demonstrated that an englacial thrusting model, developed for polythermal glaciers in Svalbard, best fits the available evidence. Thrusting probably resulted from longitudinal compression against a reverse bedrock slope, although a frozen snout, downglacier of sliding ice, may also have been a trigger. It is suggested that the role of ice-deformation, especially thrusting, in landform development has been underestimated, and that the englacial thrusting model may find application in the interpretation of other sites in the palaeo-landform record.