Landslide geomorphology of Cayton Bay, North Yorkshire, UK

Abstract

The geomorphology of a large coastal landslide complex at Cayton Bay, North Yorkshire, is described. The area inland of the complex is occupied by a main road and a number of properties, so knowledge of current and possible future landslide behaviour is important for planning and risk assessment. Instability at the site is associated with a number of faults that bring argillaceous Upper Jurassic rocks to sea level. These soft rocks are overlain by more resistant sandstones. The sequence is capped by a thick and variable succession of glacial sediments, comprising tills with interbedded sand and gravel lenses that were deposited during the Dimlington Stadial of the Late Devensian. In connection with a study aimed at understanding cliff instability and recession, coastal erosion and the risk to people and assets at Cayton Bay, detailed geomorphological. field mapping was conducted. This identified two major landslide systems, which include a periodically active mudslide complex at Cayton Cliff, recognized by shallow scarps and benches with occasional back-tilted blocks, and an area of dormant, deep-seated landslides at Tenants' Cliffs, which includes a number of graben and horst structures. The origins of the landslides are unclear, but probably involved a variety of processes that led to a reduction in material shear strength or increases in pore water pressures. The original failure probably occurred in the middle Holocene, around 3000 years ago, when sea level reached its approximate current position. This would have initiated erosion of Jurassic argillaceous rocks (Oxford Clay) now exposed at sea level on the coast, and probable deep-seated failure of the overlying, more competent strata (Lower Calcareous Grit). Alternatively, it is possible that retreat of the Late Devensian ice sheet triggered instability, and that the landslide complex is a paraglacial landform produced during deglaciation, around 13 000 years BP. However sea levels were then lower so erosion of the Oxford Clay is unlikely to have been a factor in its development. Contemporary instability is likely to be due to the combined effects of coastal erosion and high groundwater levels, both of which are predicted to increase in the future due to the impact of climate change, implying an increased risk to coastal assets and a need to manage and mitigate such risks. © Yorkshire Geological Society, 2006.