ON THE HISTORY OF THE LATE DEVENSIAN LAKE SPARKS IN SOUTHERN FENLAND,CAMBRIDGESHIRE, ENGLAND

A section in a gravel quarry at Somersham, Cambridgeshire, has reveale d evidence for a lake, named Lake Sparks, in Fenland during the Late D evensian substage of the Pleistocene. Varved sediments were deposited in this lake over a minimum period of ca. 65 yr. The varved clays cont ain red diamicton clasts, interpreted as dump, delivered to the area b y icebergs or floes from the ice-front in the Wash that deposited the Hunstanton Till. The lake is therefore considered a result of impoundi ng by the Late Devensian ice advance on the east coast. A small number of pale varves have a characteristic structure indicating increased c alcite deposition in the summer. They are interpreted as a result of c ooler summers with reduced gelifluction from the surrounding Jurassic (Ampthill) Clay. Such gelifluction introduced a mudflow into the varve d sequence at the southern end of the section. Pollen analysis confirm s the derivation of the clays from the surrounding Ampthill Clay. The varved clays are succeeded by fluviatile sediments related to a delta building into the lake from the north. The delta sediments show period ic influx of sand into the lake interrupted by quiet periods with the development of Chara meadows. A thin spread of fluviatile gravels succ eed the delta sediments, indicating the development of a braided river plain as the lake drained on the melting of the Late Devensian ice. T his was followed by permafrost development, with the formation of thin thermal contraction cracks and coversand deposition. Later, degradati on of the permafrost was associated with the formation of diapirs and a solifluction mantle, and incision of the fluviatile and lacustrine s ediments took place. Flandrian peat and marl later filled the valley s o formed. A radiocarbon date of 18310(-1440)+1980 yr BP from Salix lea ves in a drift mud at the top of channel sands preceding lake sediment , in a neighbouring section, confirms the relation of the lake to the Late Devensian ice advance. The significance of the Late Devensian sed iments at Somersham lies in the information they give on the timing an d variety of processes related to drainage and ice movement in the per iod before, during and after the ice advance to the Wash. A period of low deposition rate in the lake was followed by rapid delta sedimentat ion and lake drainage, with implications for climatic change.