Physical Landscape: Surficial Geology

Also On This Page:

Case Study

The surficial geology of Vermont is largely a legacy of the glaciers that receded from this area about 13,500 years ago. Glacial till, gravel, sand and clay are the most common surficial deposits that remain from the retreat of the glaciers. These deposits are smeared across hillsides, and even fill whole valleys. They may be several feet thick and create structure and form independent from the underlying bedrock.

Understanding the surficial geology of your town is critical to conducting an effective landscape analysis for a number of reasons.

Surficial sediments exposed along a stream bank

First of all, surficial deposits generally provide the "parent material" for the soils that have been developing over the last 10,000 years. The texture and chemical composition of this parent material has a profound influence on important factors such as soil productivity and moisture holding capacity, and often holds the key to understanding patterns of agricultural activity and plant community distribution. In addition, surficial deposits are often quarried for their commercial value (i.e., sand and gravel), and, because of variations in depth and drainage, often control where development can (and cannot) occur. For example, sand deposits (often originally deposited as deltaic and shoreline environments) generally make for poor agricultural soils because they provide scant nutrients and don't retain adequate moisture, yet are often prime for development because they easily excavated and eminently suitable for septic drainage.

Glacial till, a jumbled and unsorted mix of different sediment sizes (ranging from clay to boulder), is by far the most common surficial deposit on the Vermont landscape. Originally scraped from the bedrock and redistributed on the landscape under the advancing ice sheet, till (ironically derived from the Scottish term meaning "stubborn land") tends to give birth to soils that are rocky and dense, and often somewhat poorly drained. The rocky and often acidic nature of these soils often led 19th century Vermont farmers to abandon them in favor of soils that were more fertile and easier to plow.

In the lowlands of Vermont (i.e., the Champlain Valley and the major river valleys), till is commonly mantled by finer sediments, such as sand, silt, and clay. This pattern is a manifestation of the sorting power of flowing water. As the glacier melted away from the landscape, tremendous amounts of glacial meltwater flowed through the recently uncovered till, and carried and sorted the component particles varying distances from their source, depending on the size of the particle and the energy of the flow. Cobble-sized rocks require very fast, high energy flows to move them, while smaller sand particles can be easily transported by fast flowing water, but are quickly deposited when the rate of flow slows, such as when a flowing river meets a still lake. Microscopic clay particles, however, will often be carried long distances into still, deep water. When these finer particles are found at the surface of the landscape, it is often evidence that a glacial lake covered the area at the end of the Wisconsin Glaciation.

Sometimes surficial sediment is exposed in road cuts or riverbanks, but usually the best way to explore the surficial sediments of your area is to start digging. You'll want to look for transitions in soil color and texture, notice the shape and size of any rocks that you encounter, and depth and nature of plant roots. As always, keep your eye on the surrounding features - natural and human-made.

Shelburne's Surficial Geology

The surficial geology of the Town of Shelburne, which is dominated by silt, clay, and sand, is a reflection of the fact that it's landscape was completed covered by Glacial Lake Vermont immediately following the retreat of the Laurentide ice sheet. This vast glacial lake covered the Champlain Valley to elevations 600 feet above present day sea level, and its presence led to the mantling of the previously deposited till with

Exposed glacial till

deep deposits of fine silts and clays. Large swaths of marine sand cover these lake bottom sediments in general area between Route 7 and spear street, a manifestation of the shoreline of Champlain Sea (an inland arm of the Atlantic Ocean) that inundated the Champlain Valley to elevations 320 feet above present day sea level for several hundred years following the retreat of the ice sheet. Interestingly, the underlying glacial till is exposed in areas where streams have down-cut through the overlying sediments. (see photo at right) This till is largely composed of the local shale, limestone, and quartzite that makes up the bedrock of Shelburne. Pluvial deposits derived from slowly decomposing organic matter form the parent material for wetland soils found in the vicinity of Shelburne Pond and the LaPlatte River.

See surficial geology map of Shelburne