Subglacial meltwater channels at Lymm Dam, Cheshire

Abstract

Small-scale erosion features in the lowlands of Britain have only previously been described from the Wirral, Cheshire. In this paper a suite of similar erosion forms located within a subglacial meltwater channel is described and compared with those previously reported. The forms consist of almost linear channels incised into soft Triassic sandstone bedrock to a depth of up to 1.76 m and a group of scallops occupying a vertical rock face. The channels show two preferred orientations, one parallel to the last (Late Devensian) ice flow and another transverse to the ice flow. Two possible types of erosion processes are identified: (i) erosion by phreatic (below the water table) subglacial water flow, which produced the subglacial gorge, channels orientated parallel to the ice flow, and scallops; and (ii) erosion by vadose (above the water table) subglacial water flow, which produced the channels orientated parallel to the local bedrock slope (transverse to the ice flow). The implications for Late Devensian glaciation of the area are: (i) deglaciation was accompanied by active ice; (ii) deglaciation was achieved under warm base conditions; (iii) vertical ice thickness was greater than 40 m; and (iv) the hydraulic potential gradient was possibly orientated N14°E at the formation of the features.