A Discussion on valley slopes and cliffs in southern England: morphology, mechanics and Quaternary history - The Quaternary history of the Lower Greensand escarpment and Weald Clay vale near Sevenoaks, Kent

Abstract

In the neighbourhood, of Sevenoaks Weald many of the small hills and ridges standing up to 20 or 30 m above the streams in the clay vale south of the Lower Greensand escarpment are capped by Head deposits consisting of angular chert fragments, and other stones derived from the Greensand, set in a clay matrix. These deposits extend for a distance of at least 2 km from the escarpment, forming dissected remnants of what were originally extensive sheets, inclined at gradients of about 1.5°. The available evidence suggests they are periglacial solifluction deposits of Wol- stonian age. Probably at about the same period large-scale structural disturbances occurred in what are now spurs of the escarpment; massive blocks of the Hythe Beds subsided into the underlying Atherfield and Weald Clays, and the clays were forced up at the foot of the scarp in the form of bulges. Following this stage considerable erosion took place in the vale, accompanied by re­ treat of the escarpment within embayments between the spurs. On the eroded land­ scape solifluction debris moved up to 1 km from the scarp face during the Devensian period. This deposit again consists predominantly of clay with embedded angular chert fragments. It is about 2 m thick, with a minimum gradient of a little more than 2°, and overlies brecciated Weald Clay which typically contains several slip surfaces in its uppermost layers. Landslips in the escarpment within the embay­ ments probably occurred at about the same time. Not long afterwards, in the Late-Devensian Interstadial, around 12000 radiocarbon years b.p., a soil formed of which traces can be found buried beneath a lobate soli­ fluction sheet. The lobes extend over the lower sheet for distances of 300 m from the scarp foot at an average slope of about 7°. In the subsequent Postglacial period only minor changes have taken place; some escarpment landslips have been reactivated and the streams in the vale have eroded small channels or valleys not more than 4 m deep. Based on thaw-consolidation theory, and by using measured properties of the clays, calculations are presented which provide a reasonable explanation, in terms of soil mechanics principles, for solifluction movements of the active layer above perma­ frost on slopes inclined at angles as low as 1.5 or 2°. Under temperate conditions, mass movements are possible only on slopes steeper than about 8°. The paper includes an account of the longitudinal profiles and stratigraphy of the Eden and Medway river terraces.