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Geology of Vermont


Owl's Head Mountain, Groton State Forest
Photo: P. Gale; text: M. Gale, 2000



View of Kettle Pond from the Summit of Owl's Head

The summit of Owl's Head Mountain (elev. 1958 ft) in Groton State Forest is accessible via a short hike and provides views of Peacham Bog, Kettle Pond and Lake Groton. The rock at Owl's Head is granite-one of the official State Rocks of Vermont. Granite similar to that at Groton has been quarried for years in the Barre-Montpelier area and is an important economic mineral resource for Vermont.

The knoll at Owl's Head has weathered evenly due to the uniform texture of the gray, medium grained granite. The granite may reach several miles in depth, as is estimated for the Barre granite plutons. The rock exposed at Groton was formed about 380 million years ago and cooled many kilometers down in the earth's crust. The surrounding Gile Mountain and Waits River formations are mainly gray mica-rich metamorphic rocks called schist. The granite cuts across layers in the schist and this relationship allows geologists to determine the relative ages of rocks.

The weathered surface of the rock is generally smooth, darker gray and may be covered with lichen. On a fresh surface, the rock is coarse-grained, gray and white color with minerals visible to the naked eye. Black biotite and clear muscovite are both shiny and occur as plates. Quartz is transparent and glassy and feldspar is translucent and chalky. The granite at the mountain is composed of 35% quartz (SiO2), 35% plagioclase feldspar, 25% microcline (potassium feldspar or KAlSi3O8), 4% biotite and1% muscovite (Christman, 1956). The granite at the surface is connected at depth to other granites in the area and forms a large pluton. Coarse-grained igneous rocks generally cooled deep in the ground and slowly, allowing time for large minerals to crystallize. Pegmatite dikes may be seen in the area and were probably intruded along planes of weakness or cracks in the granite. The pegmatite is also light colored and generally very coarse-grained.

There are cracks visible in the granite. The cracks are referred to as joints and tend to form a set of parallel planar surfaces. The joints dip very steeply and trend to the northwest and northeast. Water at the earth's surface can flow along these joints and travel deep into the ground. During the winter months, water freezes within the joints and pushes the rock apart by 'frost-wedging". Blocks of rock then break off the main outcrop and tumble down hill to form talus, an accumulation of broken rock. Thousands of feet of rock have been removed by erosion during the past 250 million years, and rock is continuing to be eroded today. The granite is sheeted, as is visible on the south slopes, and the effect of ice and water working along joints and fractures in the rock continues to peel off granite fragments today. Less visible is a slow weathering of the granite bedrock and the glacial materials to form sandy , acidic soils in the area. Quartz, a common rock-forming mineral, is also resistant to chemical weathering and rather than dissolving, becomes the main component of sand.

The rock has been sculpted by glacial ice as well, and erratics or boulders of granite and other rocks are found in the valleys surrounding the mountain. The large ice-sheet that covered New England scraped the rock surface and plucked blocks as it advanced over the land. Striations, deep scratches caused by cobbles dragged at the base of the ice, are even visible on the summit of Mt. Mansfield. The ice was quite "dirty" and when it melted a variety of material in the ice was deposited as sediment. This most probably resulted in a barren landscape of bare rock, sand, and gravel with little vegetation, very different from the lush woods in Vermont today. The glacial deposits at Groton are not well sorted, but contain a rather chaotic assortment of grain sizes from sand and silt to boulders. In the valleys, much of the finer grained sands have been washed away leaving large boulders stranded on the surface.

The recent history of the past 10,000 years or so, has been landscape development in response to seasonal weather, floods, major storms, and human land use.

 

 

Generalized Geologic 
	Map of Vermont - 1970 - click for larger map image

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