This hand colored lanternslide shows a black car on a very muddy road. There is standing water on the road. Trees line the side of the road, but most of the landscape is cleared. There is a wire fence along the right side of the road and another car in the distance. This is spring? as there are no leaves on the trees but lots of mud. There are mountains in the distance. To see this image in the database, click here.
In this module, we will have a brief look at the geology of Vermont as it pertains to the landscapes on which we live.
Broadly speaking, Vermont geology can be broken into two categories. First, there is the bedrock that underlies the state. This rock varies in age and character but most of it is hundreds of millions of years old. Second, there are the softer, unconsolidated sediments that overlie the rock, these sediments are much, much younger. Most sediments date to the last incursion and retreat of the continental glacier which crept out of Canada and overwhelmed the entire Vermont landscape.
Let’s consider the rock first. The rocks of Vermont vary in character and in origin. You can see a map of Vermont’s rock types by clicking here. Rocks of the Champlain lowland and western Vermont are sedimentary, the products of long-ago eroded mountains. These sediments, now turned to rock, have been at most changed only slightly by heat or pressure since they were deposited. In central and eastern Vermont, many of the rocks are metamorphic, having been altered by heat and pressure at depth during one or more of the mountain-building events that affected Vermont hundreds of millions of years ago. There are some igneous rocks that intruded as magma into both sedimentary and metamorphic rocks. The best known are the granitic rocks of the Barre area.
To learn more about the geologic history of Vermont, try
Geologic history of Vermont and the Champlain Valley
Vermont Geologic Survey
USGS short Vermont geologic history
Let’s have a look at some of the rocks that make up Vermont using images in both the Landscape Change Program and Perkins Museum archives. We’ll start with schist, a metamorphic rock that began as ocean bottom sediment before its minerals and texture were altered by heat and pressure deep in the Earth. Here’s a schist cobble in which you can see the mica-rich, shiny matrix and the veins of quartz. Schist dominates the mountains of Vermont. Check out these examples from Camels Hump and Worcester Mountain.
Another metamorphic rock found in Vermont is marble. Marble is white and shiny reflecting the grains of calcite that form the rock. The marble industry in Vermont is well represented in the Landscape Change Program archive. Check out these images from Dorset, Proctor, Pittsford, and Danby. Slate, an important metamorphic architectural stone because it cleaves into flat plates, is also found in Vermont. The archive holds many slate-related images including some from Fairhaven, Montpelier, Pawlet, and Poultney.
The most common igneous rock in Vermont is granite and other rocks similar to granite. Granitic rocks are formed when magma (molten rock) rich in silica cools slowly underground. Check out images of granite mining, milling and transport in the archive from towns such as Barre, Hardwick, Woodbury, Granite was used all over Vermont and indeed all over the nation for a building stone. See these examples from the Champlain Islands (where there is no granitic rock!) as well as places farther afield.
Sedimentary or much more lightly metamorphosed rocks dominate western Vermont. The Monkton Quartzite is a very hard rock made almost entirely of quartz. It is commonly used as a foundation stone and can be found along Lake Champlain. Limestone is also found and mined in western Vermont and used for building. Here’s an example from Colchester and another from Isle LaMotte. Mining for metals has never been widespread in Vermont. However, there was copper mining in the eastern part of the state. Miners extracted pyrite which had associated with it, copper bearing minerals such as calcopyrite. There are numerous images in the archive from the town of Vershire (try a search for copper or for the town of Vershire), where the mining occurred. You can see the landscape effects which were significant due both to clear cutting for fuel to roast the ore and probably also due to the acid emissions from the roasting beds. For a discussion of the continuing environmental effects of the mines, click here.
Onto and over these rocks are etched and draped evidence of the glaciers that repeatedly covered Vermont. These glaciers grew and expanded in response to cooling climate over the past several million years. The glaciers eroded and smoothed rock, leaving the rounded forms you can see all over Vermont, including this example from Bolton. Looking more closely at such glacially affected outcrops, one might expect to find striations, or scratches and grooves left by the ice.
When the glacial ice melted, it left large amounts of sediment on the landscape. That sediment is a very important part of the Vermont landscape. For example, water melting from the glacier carried and deposited large amounts of sand and gravel. These deposits are used extensively for road building often after they have been sorted to size. Upland agricultural fields are often studded with large boulders. These are glacial erratics, carried by the glacier and left in place when the ice melted away.
Fine grain material, including clay and silt left behind by the glacier affects every Vermonter’s life. These very fine materials settled out in areas where lakes existed in the past, lakes that later drained, usually as the ice that dammed them melted away. In the past, clay was an important economic resource with many brickyards using it as a raw material. Road cuts expose this material but its most important effect on the landscape is its ability to make that famous Vermont material, mud. The mud makes roads difficult to navigate; many springs, trapping cars and horses alike in the days before paving.
After the glaciers left, the Vermont landscape continued to change. In the hundreds of years it took for trees to recolonize the barren hillslopes, great volumes of sediment eroded and filled valley bottoms. Erosion has continued at a slower pace ever since with landslides moving material down slope and rivers cutting through sediment that choked their valleys. Floods have carved new river channels and left sediment and debris in places it never used to be. Snow blankets the landscape every winter, reminding us of the our glacial heritage and the efforts we go to living in this climate and on this landscape. Indeed, the Vermont landscape is dynamic and ever-changing.