SURFICIAL GEOLOGY OF THE JEFFERSONVILLE 7.5-MINUTE QUADRANGLE, NORTHERN VERMONT

We present a detailed map and cross-sections depicting the surficial geology of the Jeffersonville 7.5-minute quadrangle based on 4 months of field work conducted during the summers of 2000 and 2001 with support from both the STATEMAP program and Research Experience for Undergraduate funds administered by the American Association of State Geologists. The Jeffersonville Quadrangle lies with the Green Mountains and is traversed by the Lamoille River valley. The surficial geology of the quadrangle is dominated by both till and lacustrine sediments deposited by the late Wisconsinan advance of the Laurentide ice sheet and a series of glacial lakes that occupied the river valleys during retreat.

Striation measurements at high elevations all record the NW to SE flow of the ice sheet, a direction that cuts across the N-S alignment of most ranges in the Green Mountains. At lower elevations, striae are aligned parallel to the Lamoille river valley, indicating the topographic control exerted on the thinning tongue of the ice sheet. The most common till at the lower elevations is relatively loose, has a fine sand or silt matrix, and contains only sparse erratics that are cobble size and larger. This till overlies deformed lacustrine sediments and was deposited during a readvance episode that may correspond to the well-documented Littleton readvance in the Connecticut River valley to the east and the Middlesex readvance to the south dated at 11,900 C-14 ybp or ~13,950 calb. ybp (Larsen, 2000)..

The bedrock valley of the Lamoille River and smaller tributaries are deeply buried beneath ice-contact, lacustrine, and modern alluvial sediments between the bedrock dam in Fairfax and Ithiel Falls in Johnson. An esker system (esker ridge and associated subaqueous fan deposits) exposed in the village of Johnson continues west along the Lamoille valley, but is deeply buried. Water well logs have allowed us to partially trace this esker and to document its utility as a high-yield, although hard-water-bearing, confined aquifer. Thick sequences of undeformed, varved lacustrine silt and clay fill much of the valley bottoms. Deltas indicate glacial lake levels in the valley at 1130 ft (345 m, Glacial Lake Winooski), ~800 ft (244 m, Glacial Lake Mansfield), and ~720-660 ft (220-200 m, Glacial Lake Vermont).