The term “bedrock geology” generally refers to the study of how different types of bedrock were formed, and how they came to be where they are today. In its most basic form, bedrock geology focuses on what rocks are made of, what conditions they formed under, how layers of bedrock are stacked, folded, split, or thrust in relation to one another, and what forces caused or allowed them to do so. Bedrock geology maps typically depict the locations of different bedrock types known as “formations.” A bedrock formation is one of the most basic taxonomic units used in the study of field geology. Each bedrock formation is distinguished by a characteristic grouping of rock types that have a specifically defined origin, age, structure, and mineralogy. The bedrock geology map of Williston is fairly simple in appearance, but masks the deceivingly complex story of bedrock geology in Williston and in the state of Vermont as a whole.
There are actually nine bedrock formations that underlie the Town of Williston, but the eastern two thirds of the Town are dominated by only two: the Fairfield Pond Formation, and the Pinnacle Formation. These extremely ancient bedrock formations date back at least to the Cambrian Period (500 - 630 million years ago),
Green Mountains around 450 million years ago. The other bedrock formations in Williston are significantly younger, and their juxtaposition with the older rocks is attributable to the Hinesburg Thrust Fault which snakes its way from north to south in the western part of Williston.
The image below represents a cross section of the bedrock geology near Williston, VT. The cross section runs more or less west to east, and is located just to the south of Lake Iroquois. The Hinesburg Thrust Fault, indicated on the left side of the diagram, runs roughly north to south through the western half of the Town of Williston. Along this line, older and more deeply buried bedrock types were pushed (from the east) up and over younger bedrock to the west, thus creating a situation where the near-surface bedrock in the eastern two thirds of Williston is significantly older and different in composition than the near-surface bedrock in the western third of Williston. (This image is from the 1961 Centennial Bedrock Geology Map of Vermont, published by the Vermont Geological Survey).
Fairfield Pond and Pinnacle Formation rocks are often described as “clastic,” which simply means that they are formed from small components of other rocks that have already been broken down. For example, Pinnacle Formation greywacke is a sandstone which contains elements of quartz and feldspar, and which may also contain clay particles and pieces of other metamorphic rocks. Fairfield Pond and Pinnacle Formation rocks are not generally calcareous in nature, and while areas of mildly calcareous rock may sometimes occur in these formations, one may assume that these rocks do not typically enrich the soil around them. Streambeds, old stone walls, and exposed cliff faces in the eastern two thirds of Williston are all good places to look for examples of these bedrock formations.
Williston Bedrock Cross-Section
Stone wall opening over stream
Old Downer cellar
Ledges on Gramma Ridge