Groundwater sapping and valley development in the Hackness Hills, North Yorkshire, England

This paper describes evidence for the role of groundwater sapping and seepage erosion processes in the development of valleys which cut the southern edge of the Hackness Hills plateau in North Yorkshire, England. The development of drainage in this region has previously been suggested to relate to erosion by Late Devensian sub-aerial glacial meltwater channels. The role of groundwater erosion is investigated through a combination of geomorphological studies, lithological logging and X-ray diffraction (XRD) analyses. The geology of the region consists of a series of permeable Middle and Upper Jurassic lithologies (the Corallian sequence and Lower Calcareous Grit) which overlie the impermeable Upper Oxford Clay. The rocks dip gently to the south at between 1° and 4° and are relatively unfolded. Valleys exhibit many characteristic features of groundwater sapping networks. They rise abruptly at the edge of the plateau with amphitheatre-like valley heads, alcoves in headwalls, steep bedrock side walls, flat floors, spring sites and seepage zones in many valley flanks. Lithological logging indicates that sites of groundwater emergence usually occur either at or slightly above the boundary of the Upper Oxford Clay and Lower Calcareous Grit. XRD analyses of bedrock samples indicate that seepage occurs within siltstones which contain no clay but a variable percentage of calcite. The cause of groundwater emergence is attributed to decreasing grain size and increasing calcite cementation within bedrock which combine to reduce permeability. Development of valleys in the Hackness Hills is suggested to have occurred by a combination of headward erosion by groundwater sapping processes operating in an up-dip direction superimposed onto a valley morphology shaped by surface fluvial erosion.