ICE FLOW DIRECTIONS, ICE-CONTACT ENVIRONMENT, AND THE EARLY HISTORY OF GLACIAL LAKE MEMPHREMAGOG IN NORTHERN VERMONT

The glacial geology of the upper Missisquoi river valley of northern Vermont has not been studied since reconnaissance work was completed for the state surficial map in the 1960's. The present work aims to better understand the history of ice-retreat and proglacial lakes across northern Vermont and its connection to ice-retreat down the Lamoille River valley.

Glacial striations measured along the ridgeline of the Green Mountains between the Canadian border and the Lamoille River (~80 km) and in adjacent areas east of the Green Mountains confirm that regional ice-flow was directed consistently to the southeast, across the NNE–SSW trending ranges of the Green Mountains. Younger cross-cutting striations occur at lower elevations and are always parallel to the valleys they occur in indicating that ice-flow was guided by topography as the ice sheet thinned. In this part of northern Vermont there is no evidence of ice sheet movement from northeast to the southwest.

An esker system and overlying ice-proximal and ice-distal lacustrine sediments dominate the surficial geology of the upper Missisquoi River valley. This segmented esker can be traced northward for ~50 km from a very large Glacial Lake Winooski delta bordering the Lamoille river valley to within 4 km of the Canadian border where it is buried by lacustrine sediments. This long north-south esker system and approximately north-south trending striations that cross-cut older northwest-southeast striations indicate an active, northward retreating ice front east of the main range of the Green Mountains concomitant with an active tongue of ice retreating west, down the Lamoille river valley. Limited areas of kame and kettle topography bordering the esker indicate that ice stagnation was local and did not occur over broad areas of northern Vermont.

During northward ice retreat the upper north-draining Missisquoi river valley was flooded by an early stage of glacial Lake Memphremagog. Preliminary observations of deltas indicate that the elevation of some of these early lake stages lay between 280 and 306 meters. Insufficient data were collected to draw new isobases of isostatic uplift.