Solifluction terraces in South Wales

Abstract

A hypothesis is proposed to explain the formation of certain terrace deposits in the upland valleys of South and Central Wales. It is suggested that these terraces were originally formed during the late Würm or post-Würm periglacial phases when the substratum was permanently frozen, but thin layers of surface debris moved downslope through periodic freezing and thawing. Solifluction commenced first on the warmer south- and west-facing slopes and occurred later on the colder north- and east-facing slopes. The resulting material accumulated on the valley floor and tended to move gravitationally down-valley, a tendency later intensified by meltwater and subsequently by normal stream erosion, causing deformation and, locally, the eventual disappearance of the terraces. Several stages in the development and deformation of terraces can be discerned, but the remains of terraces below south- or west-facing slopes are generally more prominent than those at the foot of north- or east-facing slopes. Steeper south- and west-facing valley sides possibly resulted from more intense physical weathering on these slopes and the rapid removal of debris to the lower parts by solifluction during multiple periglacial phases. The terraces now seen are merely remnants of the last periglacial phase.