Upper Pleistocene glacial valley-junction sediments at Pias, Trevinca Mountains, NW Spain

Sediments at Pias (Galicia) provide evidence of Upper Pleistocene glacial activities at a valley junction in the north-western mountains of Spain. The sedimentary sequence consists of lower, predominately fine-grained lacustrine deposits with few lonestones, overlain by poorly sorted, sandy gravels interstratified with massive diamicton deposited during mid-Weichselian times (MIS 3) (marine isotope stage 3). The lacustrine sediments were deposited in a glacial valley temporarily dammed by a confluent glacier. The presence of active ice is suggested by massive diamicton layers best interpreted as till, rafted sediments in lacustrine deposits and deformation structures indicative of loading and kettle formation. Frozen ground conditions are suggested by a few involution-like structures. The Pias area contains one of the few western-Spain sedimentary records of Upper Pleistocene glaciation at relatively low latitude (about 42°N) and low altitude (less than 1000 m a.s.l.). A southern dip of the Polar front to 40–45°N latitudes, as occurred during Last Glacial Maximum, could have cut moisture to the northern Fennoscandinavian ice sheets. At the same time, however, sufficient precipitations would have persisted in north-western Spain to sustain extensive ice caps and their outlets to elevations as low as c. 900 m a.s.l.