SURFICIAL GEOLOGY OF THE JOINER BROOK AND WINOOSKI RIVER VALLEYS, BOLTON, VERMONT

This report summarizes the findings from a surficial geology survey of the Joiner Brook valley and adjacent parts of the Winooski River valley in northcentral Vermont conducted in the summer of 2017. The surficial geology of this region was observed and mapped at approximately 800 field locations using the Fulcrum Mapping Application and a Lidar shaded relief map. A surficial geologic map with geologic contacts between areas underlain with differing surficial materials was created using QGIS. A geologic cross-section was constructed using both surface observations and water well data from the Vermont Agency of Natural Resources’ Natural Resources Atlas.

The town of Bolton is underlain primarily by schists and some quartzite that were metamorphosed during the Taconic Orogeny and Acadian Orogeny. Most of the surficial materials in the area were deposited as the Laurentide Ice Sheet retreated between ~13,900 and 13,700 years ago. As the ice sheet retreated to the northwest, it left behind a thick layer of glacial till, which dominates the upland landscape. These materials have been redistributed by surficial processes in the form of landslides, stream erosion, and alluvial fans. Multiple medium-sized landslides were mapped predominately in the upstream regions of Joiner Brook. The retreat of the ice sheet also blocked the flow of the westward-flowing Winooski River and south-flowing Joiner Brook. This allowed for a series of proglacial lakes (Glacial Lake Winooski, Glacial Lake Mansfield, Glacial Lake Vermont) as well as several high-elevation glacial lakes to develop, and for sediments to be deposited in these valleys in a lacustrine environment. The lower elevation Winooski River valley is dominated by these lacustrine sediments and more recent alluvial deposits.