TECTONICALLY INFLUENCED GLACIAL EROSION, AND ENSUING VALLEY INFILL - A GEOPHYSICAL SURVEY

A geophysical survey has been made of the buried door and infill of th e Gilpin-Kent Valley, south Cumbria. The 18 km valley has been investi gated using 50 seismic refraction spreads and 140 gravity stations. Th e survey has been integrated with data from boreholes and with the adj oining Morecambe Bay Barrage feasibility survey. Five geophysical cros s-sections of the valley are given, together with one from a former si te investigation near Arnside. Maximum depth to bedrock on each profil e lies between 40 and 90 m. The rock floor is dissected by faulting in to segments of Silurian and Dinantian (Carboniferous) strata; the soft er Silurian rocks have been preferentially eroded to form subglacial t unnel valleys with relief of up to 40 m. In the south the strike of th e Carboniferous beds and the trend of some of the faults swing from no rth-south into a NE-SW direction, which governs the course of the Rive r Kent to its mouth at Grange-over-Sands. The valley fill consists mos tly of silty clay, with some peat; beneath the clay is a horizon of gr avel/cobbles that is up to 40 m thick in some of the tunnel valleys. T ill has not been recognized in the survey; the gravel may be its rewor ked remains. The deep and irregular rock floor of the valley points to a glacial regime of vigorous erosion, both by ice and by melt water. Geotechnical data from boreholes have been correlated where possible, with seismic velocities and density.