STYLE OF ICE SHEET RETREAT ACROSS NORTHEASTERN VERMONT: WHAT HAVE WE LEARNED FROM DETAILED SURFICIAL GEOLOGIC MAPPING?

Surficial geologic mapping, sponsored by the Vermont Geological Survey (VGS) and funded by the USGS StateMap program, has focused broadly on central Vermont over the last 20+ years. Recently, VGS-sponsored mapping has shifted to northeastern parts of the state, areas approaching and along the Vermont’s northern border with Québec. This work provides the basis for understanding the glacial history of a region that has received relatively little detailed geological attention. The distribution of glacially deposited materials and landforms in turn provides the basis for understanding the chemistry of the region’s soils, groundwater resources, and landslide hazards.

Recent mapping has largely occurred within the north-flowing Lake Memphremagog drainage basin. Following regional SE-directed ice flow, the thinning ice sheet was redirected generally southwards, parallel to the regions’ valleys. Subglacial drainage systems in these valleys deposited both eskers and significant volumes of related ice-contact sediments. Along the ice margin a variety of moraines formed. Hummocky moraines are common across the region’s drainage divide, but have not been observed in similar settings elsewhere in the state. Other moraines may be correlative with the Littleton-Bethlehem moraine complex of northern New Hampshire.

A myriad of glacial lakes formed in tributary valleys. These lakes grew and merged as the thinning ice sheet allowed water to escape from high-level lakes into adjacent lower-elevation lakes. Flights of abandoned channels record the drainage pathways along the former ice margin between adjacent tributary valleys. In other areas abandoned channels appear to mark areas where water flowed both out of and later back onto or into the glacier. By the time the ice sheet margin approached the Québec border these smaller lakes had merged into a single large lake, Glacial Lake Memphremagog, that continued to grow as the ice sheet retreated into Québec. Most valleys contain a predictable fining-upward sequence of glaciolacustrine sediments recording deposition in a progressive ice-proximal to ice-distal environment. However, this stratigraphy is often interrupted by beds of diamict that were likely deposited as debris flows both while the lakes existed and immediately following their drainage.