RECENT SURFICIAL MAPPING IN THE SOUTHERN WORCESTER MOUNTAINS OF CENTRAL VERMONT AND EVIDENCE FOR POSSIBLE READVANCE

Surficial geologic mapping has been conducted in the towns of Waterbury and Middlesex. The area includes part of the valley of the Winooski River and the watersheds of two tributaries; Thatcher Brook and Great Brook. We have uncovered evidence for a previously unrecognized glacial lake and evidence to support the concept of a late Wisconsinan glacial readvance in central Vermont.

The most widespread surficial deposit is dense, silt-matrix till. Lake bottom, shoreline, and delta deposits of a series of down-stepping late-glacial lakes have the following shoreline elevations: Lake Thatcher, 1230 feet; Lake Winooski, 1010 feet; Lake Mansfield I, 730 feet; Lake Mansfield II, 650 feet. The newly recognized Lake Thatcher was impounded between ice to the west and hills to the east and south and drained south via Middlesex Notch. An extensive sand and gravel deposit south of Middlesex Notch may have been deposited by outflow from Lake Thatcher, and collapse features suggest deposition on a remnant tongue of ice in the Winooski River valley at a time coeval with glacial Lake Winooski. Other deposits in the map area include coarse-grained esker sediments that underlie fine-grained Lake Winooski sediments northwest of Middlesex, fan-terrace deposits, alluvial fans, and stream terrace and modern alluvial deposits.

A ditch along the north side of Perry Lea Road in the southern Thatcher Brook watershed exposes >430 horizontal feet of dense till that contains an unusual pattern of fractures. It is unclear whether the fractures are of glaciotectonic, periglacial, or other origin.

Two sites in the area display evidence of ice readvance. A prominent bench on the west flank of the Worcester Mtns. is underlain by approximately 200 feet of Lake Thatcher deposits consisting of varves overlain by sand and gravel that are in turn overlain by several feet of dense, silt-matrix diamict. The diamict is interpreted to be a readvance till. In the adjacent Great Brook watershed, eleven feet of a similar diamict overlies lacustrine material on a bench west of the brook, at 1120 feet. Evidence at several other sites is suggestive of readvance, such as deformed sections and diamict over lacustrine material, but these require further investigation. The readvance sites described here may correlate with the 11.9 ka 14C Middlesex Readvance.