The micromorphology of paraglacial and periglacial slope deposits: a case study from Morfa Bychan,West Wales,UK

Micromorphological analysis of the crudely stratified diamictons exposed in coastal sections at Morfa Bychan, west Wales, is presented and discussed. Three major units, Yellow Head, Blue Head and Brown Head, have been recognised, and their crudely stratified nature, plus clast orientations, indicate that they were all subject to mass movement down the flanks of the adjacent coastal hills. Earlier workers have suggested a periglacial environment for this mass movement, but recently it has been proposed that the Blue Head, at least, may have accumulated in a paraglacial environment through gravitational redistribution of recently deposited unstable till. It is shown here that the Yellow Head contains occasional dense silt-rich coatings on the upper surfaces of sand and gravel grains, although other cryogenic micromorphological phenomena were not observed. The Blue Head, however, contains no clear cryogenic micromorphological features, although there is abundant evidence for sediment shearing and grain rotation within the matrix. The Brown Head contains diffuse grain coatings and lenticular aggregates very similar to those observed elsewhere in periglacial solifluction deposits. Thus it is concluded that accumulation of the Blue Head was rapid, and took place in a paraglacial environment as a result of mudflow. The Yellow Head may represent periglacial solifluction immediately prior to glaciation, whereas the Brown Head accumulated above the Blue Head as a result of periglacial weathering of the adjacent exposed hill tops, and subsequent downslope solifluction of the weathering product. © 1998 John Wiley & Sons, Ltd.