GLACIAL GEOLOGY OF THE LAMOILLE RIVER VALLEY NEAR JEFFERSONVILLE, VERMONT

The Lamoille River valley cuts east-west across the Green Mountains of northern Vermont and is joined by the north-flowing Brewster River at Jeffersonville, Vermont. Detailed mapping in the area reveals the following preglacial and glacial history. In the Brewster River valley we interpret a highly weathered, red-orange saprolitic diamict to be a pre-Wisconsinan till(?) weathered during at least the last interglacial period. This is overlain by a dense, unweathered gray till deposited during the late Wisconsinan by the Laurentide ice sheet. During ice sheet retreat north, down the Brewster River valley, a high-elevation (2170 ft, 662 m asl) lake formed, draining through Smugglers Notch. The ice sheet readvanced, deforming the lacustrine sediments and depositing a loose sandy or gravely till. This readvance may be correlative with the Middlesex readvance in the Winooski River valley.

Glacial lakes were dammed in both the Brewster and Lamoille River valleys during the final retreat of the ice sheet northward and westward, respectively. The earliest lake in the Lamoille valley, east of its junction with the North Branch, reached an elevation of at least 1130 ft (345 m). This lake is most likely Glacial Lake Winooski which extended north into the Lamoille River valley from its outlet south of Williamstown, and as far west as the confluence of the Lamoille and North Branch Rivers. High-elevation lacustrine sediments aren’t found west of the confluence suggesting that the ice margin in the Lamoille River valley was at the confluence when the ice margin in the Winooski River valley retreated to Jonesville uncovering the Hollow Brook threshold. Younger and lower elevation lakes, originally documented by Connally (1968), [~800 ft (244 m); ~720 ft (220 m)] formed after the ice retreated west of Jeffersonville. A detailed measured section of silt/clay couplets grading into medium to fine sand/clay couplets along the Brewster River in Jeffersonville records the last 160 years of lake history recorded in the valley.