HISTORY OF GLACIAL LAKE LEVELS IN THE BLACK RIVER VALLEY, MEMPHREMAGOG BASIN, NORTHERN VERMONT

During the Late Wisconsonan the margin of the Laurentide ice sheet retreated into northwestern Vermont. At this time glacial lakes reached their maximum in the Memphremagog basin, and the Lamoille and Winooski River valleys. The glaciolacustrine sediments deposited in the Black River Valley during this and succeeding phases of deglaciation record the history of glacial lake levels in northern Vermont.

The study area is located 45km south of the Canadian border along the Black River Valley. The valley floor, which lies at ~268m, is covered by modern alluvial deposits and organic material, while upland areas above ~330m are blanketed with either clay-rich lodgement or sand-rich ablation till. Numerous glaciolacustrine and ice contact deposits were mapped between 270m and 300m on the valley sides. Stuart and MacClintock (1969) reported similar features in their regional study of the surficial geology of Vermont, however, the results of my study differ from theirs in several important ways.

An extensive complex deposit of glaciolacustrine varved clays and distal deltaic sands was mapped between 300m and 408m along the watershed divide between the Black and Lamoille Rivers. The location of this deposit suggests that the waters of high-level glacial lakes in the Memphremagog Basin and Lamoille River Valley were joined at this location, forming one large lake that extended from the Canadian border south to Waterbury, and potentially as far south as the town of Roxbury in central Vermont. Although there were no direct indicators of the shoreline elevation of this lake within my study area, I was able to established lower lake stages of approximately 336m and 276m based on the elevation of a series of deltas and kame deltas. When corrections for isostatic rebound at a rate of .9m/km (Larson et al., 2003) were made, these levels correspond well with shoreline elevations established by Larson and others (2003) for the lake occupying the Winooski River valley. Based on this correlation, together with the 270m elevation of the Black River Valley at the Black/Lamoille watershed divide near Lake Elligo, I conclude that glacial lakes in the Memphremagog and Mansfield Basins (Black, Lamoille and Winooski River Valleys) were joined throughout the greater part of their existence.