GLACIAL GEOLOGY OF THE PICO PEAK REGION, CENTRAL GREEN MOUNTAINS, VERMONT

The Pico Peak region straddles the Green Mountains at the latitude of Rutland Vermont. Recent mapping provides a basis for interpreting deglacial histories on both the east and west sides of the mountains that are similar in style, but differ because of local geography. Ice-marginal channels, preserved along the western slope of the mountains, mark the position of the ice sheet as it thinned. Segmented, meandering eskers were mapped in valleys bordering both sides of the mountains. The western esker was one of many draining the larger and thicker Champlain lobe of the ice sheet whereas the eastern esker occupied the sole subglacial drainage channel in the much narrower and thinner Ottauquechee river valley. Similarly, small glacial lakes were dammed by the retreating ice on both sides of the mountains, specifically in the Mendon Brook valley (outlet elevation 542 m) along the western slopes and in the South Branch of the Tweed river valley (outlet elevation 408 m) on the eastern slope. By contrast, the south-flowing Ottauquechee river valley is completely filled with glaciofluvial sediments.

Glacial till is the ubiquitous surficial material in all high elevation areas. New landslide exposures created during Tropical Storm Irene show that till was extensively remobilized in this mountainous terrain. Specifically, good exposures reveal distinct units of poorly layered diamict overlying lodgment till. These are interpreted to be debris flow deposits and may be common at the base of steep slopes yet difficult to distinguish from till in most exposures. Multiple debris flow deposits of varying thickness comprise a large percentage of the sediments deposited in a small, steep-sided glacial lake that occupied the Mendon Brook valley on the west side of the mountains. The frequent debris flow deposits suggest very unstable slopes. The larger debris flows extensively deformed the underlying lacustrine sediments.

Alluvial fans are extensive and occur on a variety of scales. The larger fans in the Ottauquechee River valley extend completely across the valley forcing the the river against the opposite valley side. Most of the mapped fans were active and received a new layer of coarse sediment during Tropical Storm Irene.