Wisconsin

Group of Quaternary glacial deposits in North America, stratified above Sangamon interglacial deposits (see Table 7, page 420). The Wisconsin stage includes tills, sands and gravels, which blanket large areas and were deposited by icesheets flowing out from centres in the Laur-entide shield of Canada. Outside the ice-sheet margins is a sequence of Wisconsin proglacial and periglacial deposits. Most of the Wisconsin deposits can be dated by radiocarbon and the stage is broadly correlated by this means with the Weichselian of northwest Europe and the Devensian of Britain. All these formations represent one cold stage, which lasted from cl00,000 bp to 10,000 bp and directly preceded our present warm stage, the Holocene or Flandrian. Detailed correlation between north America and Europe is, however, difficult. The Wisconsin is best known from the sequence of deposits around the Great Lakes Huron, Erie and Ontario. Unlike the Devensian of Britain, icesheets were present in North America throughout the Wisconsin cold stage. Fluctuations of the ice-sheet edge have been reconstructed by a study of the tills. The sequence has been divided into Early Wisconsin (clOO,GOO-53,000 bp), Middle Wisconsin (53,GOO-23,000 bp) and Late Wisconsin (23,00010,000 bp). Periods of glacial advance are seen as stadials. The ice-sheets reached their maximum extent during the Late Wisconsin phase. Outside the margin of the ice-sheets, organic and lacustrine deposits have provided evidence for four major interstadials: the St Pierre (date uncertain, but beyond the range of radiocarbon), Port Talbot I (date uncertain), Port Talbot II (48,000-42,000 bp), Plum Point (date uncertain).