The degradation of the Barton Clay Cliffs of Hampshire

Abstract

Summary The Barton Clay crops out along a 4·8 km stretch of the Hampshire coast and together with its capping of Plateau Gravels forms a cliff profile averaging around 30 to 37 m high. The original classic work on the geology of the Barton Clay had suggested a total thickness of only 34·5 m; more recent work involving boring and levelling on the cliff face has shown the formation to be 46·4 m thick. Active degradation of the outcrop creates a markedly stepped profile, relatively flat bench levels being separated from one another by steeper scarp slopes. The benches are underlain by up to 6 m of slipped debris or bench rubble. The scarps are formed by exposures of the solid Barton Clay and Plateau Gravel in the case of the top scarp. The interface between the bench rubble and Barton Clay is formed by a shear surface, an underground continuation of the scarp surfaces, which has been polished and grooved as a result of the seaward movement of the bench rubble. These shear surfaces are developed along particular, identifiable stratigraphic horizons within the Barton Clay and tend to develop along these horizons irrespective of the height of the latter within the cliff profile. Variations in the morphology occur laterally along the outcrop and can be related to the erosional history and partly to the geology. Five individual processes of degradation are particularly active on the undercliffs; these comprise bench sliding which is sliding of the bench rubble over the shear surfaces, collapse of the scarp slopes or scarp slumping, wearing down of the scarp slope by weathering or scarp spalling, and erosion by mud-flows and running water. Both mud-flows and streams when well developed cut through bench rubble and into the scarp slope and locally destroy the stepped profile. The current activity of these processes is due to marine erosion of the toe, the net result of the processes being a progressive retreat of the cliff top averaging over one metre per year in some places.