This article, by geologist Peter Thompson, appears in the Summer 2024 Long Trail News, titled “Geology of Vermont’s Southern Long Trail.”
Vermont’s Green Mountains were formed over hundreds of millions of years, in three major events called the Grenville, Taconic, and Acadian Orogenies, all more than 350 million years ago. An orogeny is a mountain-building process caused by enormous tectonic plate shifts and collisions, which alter the original rock formations.
In Vermont, bedrock layers of rock were compressed, deformed and chemically changed under great pressure and high temperature, allowing metamorphic minerals like biotite and garnet to grow. The rocks in the southern Green Mountains are generally older than those in the north.
Today’s Green Mountains reached their basic shapes through eons of erosion and uplift. Then, during the last ice age, roughly 100,000 to 13,000 years ago, glaciers further diminished the mountaintops, smoothed bedrock surfaces, steepened south-facing slopes by plucking, and filled lower areas with glacial till.
These photos, taken on the Long Trail from Massachusetts to the southern boundary of the Breadloaf Wilderness, illustrate the main types of materials a hiker is likely to encounter on the southern Long Trail.
Stamford Granite, southernmost Vermont
The southernmost Green Mountains are made of very old Precambrian rocks of Laurentia. This granite has been dated radiometrically to be about 960 million years old, one of the younger Precambrian rock units. The rock is composed of feldspar (pinkish to white), black biotite mica and quartz (clear to bluish).
Cheshire Quartzite on Pine Cobble, south of the Massachusetts border
Five hundred million years ago, beach sand was deposited along the eastern margin of the ancient Laurentian crust. It metamorphosed into quartz sandstone, and then quartzite. Note the planar joints and curved fractures. Because of strong internal bonds within quartz, quartzite tends to break with curved “conchoidal” fractures.
Glacial Boulder of Gneiss (north of Stratton-Arlington Road)
The Wisconsin Glacier left many boulders in its wake. These are called erratics when they differ from the type of rock underneath, meaning it was transported by the force of the glacier. In this case, the boulder and bedrock are both gneiss, so the boulder was likely not transported far.
Schist with Quartz Veins at Baker Peak (near Griffith Lake)
Mudstone and shales are metamorphosed to schist. During metamorphism, many of the chemical reactions release silica and water. They may escape to Earth’s surface, but more commonly they crystallize as veins and pods of quartz where conditions favored precipitation.
Cheshire Quartzite at White Rocks (near Wallingford)
The steep surface of the cliff is a bedding plane, which rotated during mountain-building from near horizontal to about 45 degrees toward the west.
Weathered Marble Breccia north of the New Boston Trail
Marble is rare along the Long Trail, but it is abundant in the Champlain Valley. Marble is metamorphosed limestone. Both marble and limestone are soft and easily weathered by chemical reactions.
Peter Thompson is a geologist who co-edited the 2011 Bedrock Geologic Map of Vermont. He and his wife Thelma mapped bedrock in the northern Green Mountains from 1986 to 2000. Since then they have been hiking the Long Trail south to north watching for interesting geologic features. They are retired from teaching at Cornell College in Iowa and the University of New Hampshire.
Geological Terms
Metamorphic: Substantially changed by intense heat and pressure.
Tectonic plate: An area of Earth’s surface rocks, usually large, floating on molten material deep below the surface.
Glacial till: Mixed sediment carried by a glacier, typically deposited when the glacier melts.
Precambrian: The earliest period of Earth’s history, ending about 570 million years ago.
Laurentia: A large continental craton, or region, that forms the ancient geological core of North America.
Radiometric dating: Determining how long ago rocks were formed by analyzing its emission of radiation or its current or past reaction to radiation.
Planar joints: Flat or nearly flat planes of weakness in stone or minerals.
Conchoidal: A shell-like fracture caused when certain minerals sustain a blow, with a scalloped appearance.
Nice article. I would only add that the geological difference between Vermont and New Hampshire should be define as the succession of tectonic plate accretions from the multiple supercontinents of Rodinia and Gondwana that, like a Dagwood Sandwich, created the lithological spines of New England and Eastern North America. The moat volcanic that formed the White Mountain Batholiths and Ossipee Mountains in New Hampshire, of course is that part of the Mesozoic rifting that Split Pangea creating the Atlantic Ocean starting some 160 million years ago.
Victor C. Capelli
Environmental Educator, CALS,
Bennington. Vermont, correspondent Vermont Country Magazine, 845-514-7189