MAPPING SURFICIAL GEOLOGY AND INTERPRETING THE GLACIAL HISTORY OF THE CROSSETT BROOK AREA IN NORTH-CENTRAL VERMONT

The movement and retreat of the Laurentide Ice Sheet during the Wisconsinan glaciation greatly influenced the surficial materials and landforms found in Vermont today. Surficial geologic maps are helpful in understanding the geologic past and glacial dynamics in addition to planning infrastructure, assessing hazards, and identifying economic resources. Here we present a surficial map and cross-sections of the Crossett Brook area, a tributary of the Winooski River in north-central Vermont, and interpret the local glacial history. Using modern mapping methods we update mapping completed in the 1960s.

We recorded about 750 field observations of surficial materials and landforms using the Fulcrum mobile app. These observations, along with LiDAR hillshade, aerial imagery, and topographic basemaps, allowed for the interpretation of geologic contacts and construction of a surficial geologic map using GIS software. We created three cross-sections using private well log data to show the stratigraphy and bedrock depth across the area.

A layer of glacial till of variable thickness was deposited by the ice sheet and constitutes the base layer for most of the mapped area. Fine-grained sediments are common in the Winooski river valley and are interpreted to have accumulated in a series of glacial lakes that were dammed by the retreating ice sheet. These quiet-water lacustrine sediments make up a large portion of the mapped area and directly overlie glacial till. Extensive channeled terraces composed of coarse-grained sand and gravel are interpreted as deltas deposited by Crossett Brook. Post-glacial streams have cut through older glacial sediments leaving abandoned fluvial terraces and depositing alluvium. Other Holocene landforms produced by streams and erosion events include an alluvial fan and landslides. Landslide risk is highest on steep slopes of fine-grained glacial sediments directly next to streams due to the instability of sediment and erosion by streams.