MAPPING SURFICIAL GEOLOGY AND INTERPRETING THE GLACIAL HISTORY OF THE BROOKFIELD QUADRANGLE IN NORTH-CENTRAL VERMONT

Accurate maps depicting sediments and landforms are essential for performing hazard assessments, planning infrastructure, locating economic resources, and monitoring pollutant movement. Vermont surficial materials are primarily Laurentide Ice Sheet deposits and glacial lake sediments. The existing Vermont surficial geology map is low-resolution and outdated; modern observations and remotely sensed data aid reinterpretation at a finer scale. We present a detailed map and cross-sections of the surficial materials and landforms in the Brookfield quadrangle in north-central Vermont and use our observations to interpret the local glacial history.

We recorded 1,850 field observations using the Fulcrum mobile app. These observations were used in tandem with LiDAR hillshade, aerial imagery, and topographic maps to interpret geologic contacts and produce a surficial geologic map using GIS software. We used private well data to construct cross-sections showing the thickness and stratigraphy of valley fill at four areas of interest.

The local bedrock is highly weathered calcareous phyllite, preserving few glacial striations; most trend between 180˚ and 194˚ indicating ice sheet motion from north to south. A north-south valley bisects the quadrangle, outside of which a thin layer of till punctuated by bedrock outcrops is the dominant surficial material. In the valley, we mapped esker segments and earlier projects mapped this esker system both north and south beyond the quadrangle boundary. A drainage divide separates the valley in the south—occupied by Glacial Lake Hitchcock—from the valley in the north—occupied by Glacial Lake Winooski. Valley sediments fine upwards reflecting progressively lower energy environments resulting from ice retreat. Fine-grained sediment deposited in the middle of the valley grades to coarse sand along the valley walls closer to local stream sources. Partially eroded deltas at three locations south of the drainage divide indicate the elevation of Glacial Lake Hitchcock was approximately 225 m locally. Coarse gravel bars were deposited on top of lacustrine sediments south of the drainage divide suggesting a period of high-energy fluvial deposition after Glacial Lake Hitchcock drained. Holocene alluvium and alluvial fans are the most common surficial materials on the valley floor.