NEW INSIGHTS INTO THE INTERRELATIONSHIPS BETWEEN LATE PLEISTOCENE–EARLY HOLOCENE PROGLACIAL LAKE AND MARINE WATER BODIES IN THE ST. LAWRENCE AND CHAMPLAIN VALLEYS IN NEW YORK, VERMONT, AND ADJACENT CANADA

Late Pleistocene to early Holocene strandline deposits and landforms record changes in water levels and outflow of proglacial lakes and the regression of the Pleistocene‒Holocene Champlain Sea in the St. Lawrence and Champlain valleys of New York, Vermont, and adjacent Canada. We report a preliminary estimated post-glacial isostatic uplift gradient of approximately 0.98 m/km NNE based upon spatial analysis of beach ridges and deltaic sandplains that formed along proglacial lake and marine shorelines using high-resolution LiDAR elevation and hillshade models. Beach ridges are subtle features but are easily identified on LiDAR hillshade models. Unlike deltaic sandplains, they are typically synchronous and widespread, especially on windward-facing shorelines, and do not aggrade far above the water bodies in which they form. Elevation–distance plots of proglacial lake and marine strandline features reveal episodes of rapid proglacial lake drawdown (breakout floods), periods of more gradual release of impounded proglacial lake reservoirs, and the regression of the Champlain Sea.

The breakout of Glacial Lake Iroquois from the St. Lawrence Valley into the Champlain Valley initiated the rapid incision of the Lake Coveville outlet with the release of meltwater southward through the Hudson Valley. Lake level in the St. Lawrence Valley briefly stabilized at the Frontenac level, which discharged through The Gulf near Covey Hill, before the ice margin receded from Covey Hill. The Lake Iroquois breakout and subsequent drainage of Lake Frontenac caused a rapid, ~10m drop in Lake Coveville in the Champlain Valley to a poorly defined “Upper” Lake Fort Ann level. Lake Fort Ann continued to fall gradually another 30m to a well-defined “Stable” Lake Fort Ann, at which point proglacial lakes in the St. Lawrence and Champlain valleys were confluent. We consider the gradual fall of Lake Fort Ann to be the result of gradual incision of older, less stable, valley-fill deposits in the southern Champlain or northern Hudson valleys. Lake Fort Ann stabilized when incision reached a bedrock threshold near Fort Ann, New York. The lake subsequently dropped another 50 m as ice receded from the St. Lawrence valley to a well-defined upper marine limit of the Champlain Sea.