CHANGING SUBGLACIAL/ICE-MARGINAL DRAINAGE SYSTEMS DURING LAURENTIDE ICE SHEET RETREAT ACROSS NORTHERN VERMONT

Dramatic yet systematic changes occurred in the hydrology of the thinning Laurentide Ice Sheet as it retreated across northern Vermont. Detailed mapping of surficial materials and landforms on LiDAR base maps provide insights into these changing hydraulic systems. Specifically, subglacial and ice-marginal drainage systems are revealed by (1) the distribution of eskers and related esker-fed subaqueous fans and deltas, (2) sedimentary structures within those ice-contact sediments, (3) fluvially eroded bedrock, and (4) abandoned ice-marginal channels and terraces.

The oldest preserved drainage systems developed when the ice sheet completely covered the mountains and subglacial water was funneled from northwest to southeast through low passes in the Green Mountains. Similarly oriented striations along the roughly north-south ridge lines of the Green Mountains indicate that the ice sheet flowed from NW to SE obliquely across the mountains implying that the hydraulic gradient also sloped to the southeast. As the ice sheet thinned it’s flow became topographically controlled and tongues of the retreating ice sheet flowed parallel to major valleys. Subglacial water flow consequently shifted and no longer crossed drainage divides, flowing instead parallel to major valleys where extensive esker systems were deposited. This transition in ice flow and water flow across the mountains was time transgressive from south to north, but occurred rapidly as the ice sheet was thinning 9–13 m/year at this time. The esker systems were also time-transgressive, younging from southeast to northwest as the ice sheet retreated in this direction. Coevally, ice-marginal drainage systems developed in some valleys routing both meltwater from the adjacent ice sheet and meteoric water flowing from the emergent mountains. These are most extensively developed along the western flank of the Green Mountains, i.e. the eastern side of the Champlain Valley, as thinning ice in the Champlain valley retreated northwards.