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Geology of the Underhill Area, Vermont - Summary and photographs : M. Gale, 2008

Click on the images below for larger maps.

Most rocks in the world originate as deposits of sediment along continental margins and in ocean basins, as volcanic extrusions, or as intrusive rocks at depth in the earth’s crust. Mountain belts generally form at active plate boundaries where rocks are folded, faulted, and metamorphosed. Rocks exposed at the surface are generally being eroded or destroyed slowly over time by wind, water and ice. All of these processes are evident in rocks of Vermont.

The Lower Cambrian to Neoproterozoic formations in Underhill are, from oldest to youngest: the Pinnacle Fm., the Underhill Fm., and the Fairfield Pond Fm. (Photo: J. Kim) (see descriptions on the map at the left). The youngest rocks in the section are the Cambro-Ordovician Sweetsburg Fm.. The Pinnacle and Underhill formations are composed of metamorphosed sedimentary and volcanics rocks deposited during rifting of pre-Cambrian basement around 554 million years ago (554 mya age from the Tibbit Hill volcanics in Quebec, Kumarepeli and others, 1989). The rifting was associated with the opening of the Iapetus Ocean. The entire section has been affected by multiple fold and fault events (mountain building or orogeny) and accompanying metamorphism associated with the closing of the Iapetus Ocean. A variety of units, including slates, schists, phyllites, greenstones and serpentinites, were folded, stacked on top of each other by faults, and metamorphosed during closure of the ocean basin and the destruction of the ancient eastern margin of North America during the Taconic Orogeny (Ordovician - 450 mya). During the Devonian Period, the ocean basin closed completely, and the African and North American continents collided in the Acadian Orogeny (360 mya). Vermont was now part of a large continental landmass. Mesozoic (100 mya) normal faults and intrusive dikes in Vermont are associated with the opening of the present Atlantic Ocean.

The landscape today reflects the underlying bedrock, plus sculpting of the land by glaciers 12,000 years ago, surficial deposits left behind during stages of ice retreat, erosion by rivers, wind and ice, and the impact of the human community.

Looking north to Mt. Mansfield from Camels Hump. The layering in the metamorphic rock is folded or arched to form the Green Mountain Anticlinorium.

Erosion along bedding planes and fractures in the Pinnacle Formation at the Old Mill, Jericho, VT

Younger fractures cut across the older metamorphic fabrics in the Pinnacle Formation. Water may flow along fractures in bedrock. Intersecting fractures and foliation also explain the Cantilever Rock on Mt. Mansfield.

Photo: T. Eliassen
View of Smugglers Notch with the Champlain Valley and the Adirondack Mountains (pre-Cambrian rock) to the west. Fracture patterns and foliation in rock are a factor in erosion of the mountains.

Tibbit Hill volcanic rock, Fletcher, VT. Fine grained,basalt with feldspar phenocrysts was formed during rifting and opening of the Iapetus Ocean. The rock was deformed and metamorphosed during the Tacoinian and Acadian orogenies.

This outcrop of the Pinnacle Formation in Underhill,VT has a long history. Sediment from pre-Cambrian basement was eroded and deposited in a rift basin (when Vermont did not yet exist), buried and lithified in the Iapetus Ocean, deformed and metamorphosed during plate collisions, and eroded and uplifted to form part of an irregular topography. All the while, Vermont drifted north, the land was eroded by glacial ice, covered by glacial deposits, and the rock is now at the surface where it is weathered and eroded by wind, water and ice.

This map shows a portion of the Surficial Geologic Map of Vermont, 1970. Deposits in the Underhill area include: glacial till (t), glaciofluvial kame terrace (kt) and kame moraine (km), glaciolacustrine beach gravel (bg), lake gravel (lg), pebbly sand (ps), delta sand (ds), well sorted sand (ls), and bouldery clay (bc), post-glacial alluvium (al), and peat and muck (p). Exposed bedrock is shown in red.

To see the original drafted map at 1:62,500 scale, click the image above.

This map shows the bedrock geology draped on digital orthophotograph. The school is marked in red and houses are the black dots.





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