Devensian glacigenic sedimentation and landscape evolution in the Cardiganarea of southwest Wales

The depositional processes associated with late Devensian ice in areas bord ering the Irish Sea basin have been the subject of considerable debate. Amo ng the key areas around the Irish Sea, southwest Wales occupies a particula rly crucial position because it is here that ice flowing from the north imp inged upon the coast orthogonally and encroached inland. Two main hypothese s have emerged concerning deglaciation of the Irish Sea basin. The traditio nal hypothesis holds that sedimentation was ice-marginal or subglacial, whe reas an alternative hypothesis that emerged in the 1980s argued that sedime ntation was glaciomarine. Southwest Wales is well-placed to contribute to t his debate. However, few detailed sedimentological studies, linked to topog raphy, have been made previously in order to reconstruct glacial environmen ts in this area. In this paper, evidence is presented from four boreholes d rilled recently in the Cardigan area, combined with data from coastal and i nland exposures in the lower Teifi valley and adjacent areas. A complex his tory of glaciation has emerged: (i) subglacial drainage channel formation i n pre-Devensian time, (ii) deposition of iron-cemented breccias and conglom erates possibly during the last interglacial (or in the early/mid-Devensian interstadial), (iii) late Devensian ice advance across the region, during which a glaciolacustrine sequence over 75 m thick accumulated, within a gla cial lake known as Llyn Teifi, (iv) a second high-level glaciolacustrine su ccession formed near Llandudoch, (v) outside the Teifi valley, ice-marginal , subglacial and glaciofluvial sediments were also laid down, providing a n ear-continuous cover of drift throughout the area. Glacial advance was char acterized by reworking, deformation and sometimes erosion of the underlying sediments. The glaciomarine hypothesis is thus rejected for southwest Wale s. Copyright (C) 2001 John Wiley & Sons, Ltd.