Two nivation cirques near Aberystwyth, Wales
DOI:
https://doi.org/10.26485/BP/1966/15/10Keywords:
solifluction, snow melting, drift platformAbstract
In the uplands of Wales, both glacial and nivation cirques occur. In late glacial times, the former were occupied by cirque glaciers which have the ability to move debris out of the cirque. These cirques are characterised by erosion features, usually a lake basin enclosed by a moraine. The nivation cirques on the other hand were occupied by a perennial snow patch, incapable of movement. Their floors are characterised by deposition, for debris, produced by freeze - thaw, moved underneath the snow to the tower part of the snow patch and accumulated there to form a drift platform.
In the drift platform of Cwm Du, stream sections expose these drifts to depths of 50 feet (15 metres) showing beds of unsorted rock debris in a tough matrix of blue-grey silty clay alternating with more stony layers, usually yellow-grey in colour. The former are typical of the solifluction deposits on the local rock type, white the latter are interpreted as poorly washed residues of the deposit from which snow melt has washed some of the muddy matrix.
Sometimes, as in Gwm Tinwen, a protalus or pseudo-moraine occurs, usully associated with a steep high backwall so that debris from the exposed backwall, instead of lodging on the snow patch slides down its steep surface to form a ridge at its foot.
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