LATE QUATERNARY DEGLACIATION OF THE SOUTHWESTERN ST-LAWRENCE LOWLAND,NEW-YORK AND ONTARIO

The style of deglaciation and maximum extent of earliest proglacial la kes have been reconstructed for the southwestern St. Lawrence Lowland of New York and Ontario using ice-marginal sediments and landforms, st randline features, and the areal distribution of Candona subtriangulat a-bearing rhythmites. An important recessional position, the Carthage- Harrisville ice border, fronted proglacial Lake Iroquois in the easter n Lake Ontario Basin. Subsequent ice retreat, probably earlier than 12 ,500 yr B.P., allowed Lake Iroquois to expand along the northwestern f lank of the Adirondack Mountains and into the St. Lawrence Lowland. Co ntrasting styles of deglaciation, controlled primarily by water depth, resulted in a land-based ice margin which withdrew gradually off the northern slope of the Adirondack Mountains, while the ice margin in th e western St. Lawrence Lowland retreated rapidly in the deep waters of Lake Iroquois. The maximum extent of ice retreat during the early pha ses of Lake Iroquois has been estimated on the basis of the distributi on of the ostracode Candona subtriangulata in Lake Iroquois and younge r sediments in New York, and by northward projections of Iroquois shor eline elevations to the region bounded by the Madawaska Highlands (Ont ario). Results indicate that the southwestern-St. Lawrence Lowland was rapidly deglaciated during the highest phases of Lake Iroquois. The d istribution and radiocarbon chronology of fossiliferous sediments rela ting to lower post-Iroquois levels also confirm that these extensive p roglacial lakes occupied the Lowland well before the Champlain Sea inc ursion.