Development of glacial Llyn Teifi, west Wales: evidence for lake-level fluctuations at the margins of the Irish Sea ice sheet

A thick sequence of Quaternary laminated silt and clay, sand, gravel, and diamict has recently been proven at Llandudoch (St Dogmaels), west Wales. Most of these deposits accumulated in a tributary valley of Afon (River) Teifi, that formed prior to the last glaciation. The laminated silt and clay occur up to 103 m above present sea level, and are considered to have been deposited in Llyn (Lake) Teifi—a large ice-dammed lake that developed in the Teifi valley as the Irish Sea ice sheet impinged onto the Welsh mainland during the late Devensian. Rhythmic units are interpreted as annual varves, whose composition was determined by the distance from the sediment discharge point. Short-term climate fluctuations during the summer months are evident in some of the varve units. Stratigraphic variations within the laminated deposits are considered to reflect different levels of Llyn Teifi, which varied according to the position of the Irish Sea ice sheet in the Teifi valley and the elevations of three overspill channels. The overlying diamict, sand and gravel deposits are thought to have accumulated close to the ice front as it encroached into Llyn Teifi. During the final retreat of the Irish Sea ice sheet a series of kamiform sand and gravel deposits formed along the margins of the glacier.