ICE-MARGINAL CHANNELS AND THE EVOLUTION OF GLACIAL LAKE MEMPHREMAGOG ALONG VERMONT’S NORTHERN BORDER

The modern day Lake Memphremagog is a large lake straddling the Vermont/Québec border east of the Green Mountains. For over 100 years geologists have recognized that the broader lake basin was once occupied by a much larger glacial lake, Glacial Lake Memphremagog, (Hitchcock, 1906). Recent detailed mapping in areas surrounding the modern lake combined with analysis of landforms visible in LiDAR hill-shade imagery allow a detailed interpretation of processes occurring as this lake evolved during ice sheet retreat.

As the ice sheet east of the Green Mountains retreated generally north across the area, several adjacent north- and northwest-draining river valleys east of the Green Mountains were dammed forming subparallel lakes at elevations determined by their respective drainage divides. In order of decreasing elevation these lakes occupied the Upper Missisquoi, Barton, Clyde, and Black River valleys. Partial drainage of high-level lakes into adjacent lower elevation lakes is recorded by nested, large-scale abandoned channels extending between adjacent lake basins. Similar flights of abandoned channels occur on north-facing slopes between most adjacent lake basins indicating that ice-marginal channels focused most water flow during these drainage events. The elevation drop between adjacent channels ranges between 3 and 16 m and may reflect the yearly thinning rate of the ice sheet. One favorably oriented subglacial tunnel (esker) situated between two of the lake basins may have also been instrumental in routing water from the higher to the lower glacial lake. Glacial Lake Memphremagog formed when all the smaller lakes merged utilizing the lowest drainage divide in the Black River valley, currently occupied by Lake Eligo. Projections of the lake surface utilizing an isostatic tilt of 1.2 m/km to 330 closely fit geologic estimates of lake level across the lake basin based on known deltas and the mapped extent of glaciolacustrine deposits. During this time the Clyde River delta grew completely across the lake. When further northward retreat of the ice sheet into Québec eventually uncovered the Lac Nick outlet, lake level north of the delta rapidly fell ~85 m. Water ponded south of the delta eroded a channel through the delta, what is now the narrow neck separating the South Bay of Lake Memphremagog from the main lake.