POST-GLACIAL ISOSTATIC REBOUND, SENECA LAKE BASIN, NEW YORK

Retreat of the Laurentide ice sheet from the Valley Heads moraine position in central New York led to impoundment of deep pro-glacial lakes in the Finger Lakes troughs between about 16,000 and 15,000 years ago. These lakes are marked by remnant delta surfaces currently “hanging” above the modern lakes, as originally described by Fairchild in the late 1890s. Post-retreat isostatic rebound has tilted the original lake shorelines up to the N-NE. Correlation of delta topset-foreset contacts can be used to define individual lake plains, though this is difficult because the pro-glacial lakes generally dropped in elevation as thresholds were incised or lower outlets were uncovered during the northward ice retreat. However, a stable threshold existed for the Seneca Lake trough at Horseheads during much of the ice-retreat phase, which resulted in a relatively long-lived stable lake level in the Seneca basin and widespread delta formation. Thus, these pro-glacial lake deltas stranded high above the modern lake can be correlated more easily than those from other Finger Lakes basins. Yet even here, some correlations are uncertain. Preliminary analysis of topographic maps and digital elevation data, coupled with field checking of delta sediments exposed in road cuts, stream banks, and gravel pits, yields a reconstruction of this highest Seneca Lake basin pro-glacial lake shoreline along the east margin of the Seneca trough between Horseheads and Ovid. The correlation considered most likely yields a S-to-N rebound gradient of approx. 0.46 m/km (comparable to Fairchild's original data), though scattered higher deltas suggest a gradient as high as 0.83 m/km. Furthermore, the elevation data suggest, counter-intuitively, a decrease in gradient to the north, a paradox that remains to be resolved.