| Popis: |
The style of deglaciation and maximum extent of earliest proglacial lakes have been reconstructed for the southwestern St. Lawrence Lowland of New York and Ontario using ice-marginal sediments and landforms, strandline features, and the areal distribution of Candona subtriangulata -bearing rhythmites. An important recessional position, the Carthage-Harrisville ice border, fronted proglacial Lake Iroquois in the eastern Lake Ontario Basin. Subsequent ice retreat, probably earlier than 12,500 yr B.P., allowed Lake Iroquois to expand along the northwestern flank of the Adirondack Mountains and into the St. Lawrence Lowland. Contrasting 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 the western St. Lawrence Lowland retreated rapidly in the deep waters of Lake Iroquois. The maximum extent of ice retreat during the early phases of Lake Iroquois has been estimated on the basis of the distribution of the ostracode Condona subtriangulata in Lake Iroquois and younger sediments in New York, and by northward projections of Iroquois shoreline elevations to the region bounded by the Madawaska Highlands (Ontario). Results indicate that the southwestern St. Lawrence Lowland was rapidly deglaciated during the highest phases of Lake Iroquois. The distribution and radiocarbon chronology of fossiliferous sediments relating to lower post-Iroquois levels also confirm that these extensive proglacial lakes occupied the Lowland well before the Champlain Sea incursion. |
