Switching flow patterns within the last ice sheet in northern Scotland

Abstract

Synopsis A confluence zone existed within the northern sector of the last British Ice Sheet (BIS) where ice flowing from the Northern Highlands met ice streaming out of the Moray Firth and across the plain of Caithness. At Wester Clett, a complex 35m thick sequence of glacigenic sediments provides important evidence of dynamic interactions between these ice lobes that are consistent with recent computer simulations of the behaviour the last BIS. Sedimentological evidence suggests that early advance of Northern Highlands ice, probably between 33.5 and 30.5 ka, was followed by a first incursion of Moray Firth ice. Ice flowing out of Strath Halladale then deposited sands and gravels along its margin. Later, around the time of the Last Glacial Maximum, a powerful flow of Moray Firth ice diverted Northern Highlands ice to flow to the NW across the present coastline, depositing tills of mixed provenance. Cosmogenic exposure dates of c . 18 ka for summits in southern Caithness indicate that the ice sheet thinned subsequently, but flow to the NW continued. The youngest till and its associated moraine systems record a final movement of Northern Highlands ice. Subsequent ice retreat was accompanied by uncoupling of ice lobes at the Caithness–Sutherland border soon after 16 ka.