THE MICROMORPHOLOGY OF PARAGLACIAL AND PERIGLACIAL SLOPE DEPOSITS - ASTUDY FROM CASE MORFA-BYCHAN, WEST WALES, UK

Micromorphological analysis of the crudely stratified diamictons expos ed in coastal sections at Morfa Bychan, west Wales, is presented and d iscussed. Three major units, Yellow Head, Blue Head and Brown Head, ha ve been recognised, and their crudely stratified nature, plus clast or ientations, indicate that they were all subject to mass movement down the flanks of the adjacent coastal hills. Earlier workers have suggest ed a periglacial environment for this mass movement, but recently it h as been proposed that the Blue Head, at least, may have accumulated in a paraglacial environment through gravitational redistribution of rec ently deposited unstable till. It is shown here that the Yellow Head c ontains occasional dense silt-rich coatings on the upper surfaces of s and and gravel grains, although other cryogenic micromorphological phe nomena were not observed. The Blue Head, however, contains no clear cr yogenic micromorphological features, although there is abundant eviden ce for sediment shearing and grain rotation within the matrix. The Bro wn Head contains diffuse grain coatings and lenticular aggregates very similar to those observed elsewhere in periglacial solifluction depos its. 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 pri or to glaciation, whereas the Brown Head accumulated above the Blue He ad as a result of periglacial weathering of the adjacent exposed hill tops, and subsequent downslope solifluction of the weathering product. (C) 1998 by John Wiley & Sons, Ltd.