The Geologic History and Geomorphology of the

Green Mountains of Vermont

By Alyson Churchill

Figure 1: A map of the geo-physiographic provinces of Vermont, with the Green Mountain Province highlighted in the central part of the state (from Doolan, 1996).

Overview• The Green Mountains extend from Vermont’s northern to

southern border, and provide evidence for both pervasive and complex geologic activity.

• The deposition of the basement complex and subsequent sedimentation form the base of the Green Mountains.

• They record the activity of ancient plate tectonics and were heavily influenced by the Taconic and Acadian Orogenies.

• The breakup of Pangaea is recorded in the Green Mountains through evidence of extensional tectonics.

• Glaciation during the Pleistocene was very influential.• Geomorphologic processes continue to change the Green

Mountains.

Underlying Basement Complex

• Core of the Green Mountains consists of Precambrian Grenville basement rocks derived from previously deposited sediments-these comprised the ancient Grenville Mountains. • After the erosion of the Grenville Mountains, a major

rifting event resulted in multiple episodes of magmatic activity and the first significant sediment input into basins. • Formation of the Iapetus Ocean deposited marine

sediments. • A thick cover of mud was then deposited on top of the

marine sediments, with its source being deep ocean sediments to the east.

Figure 2: Simplified map of the geology of Vermont, including the Proterozoic basement rocks and subsequent depositions (from Coish, 2010).

Uplift of the Green Mountains-the Taconic

Orogeny• The Taconic Orogeny-collision of an island arc with the North

American continent. • This thrust Cambrian-Ordovician rock units onto the Laurentian

continental margin, resulting in pervasive deformation and high-grade metamorphism• A deep sedimentary basin developed after the Taconic Orogeny,

recycling the sediments of structures produced by it.

Figure 3: Diagram reconstructing orogenic activity as a result of plate tectonics during the Tectonic Orogeny. C1-C3 detail the closing of the Iapetus Ocean by the subduction of oceanic crust under a trench, with mountains developing as a result of sediment accumulation along the continental margin (from Doolan, 1996).

The Acadian Orogeny - Continuing Deformation

• The Acadian Orogeny-collision of one or more microcontinents and island arcs with North America. • This produced deformation and metamorphism of more intense

magnitudes, refolding previously deformed structures within the Green Mountains. • Large amounts of heat and pressure melted the material within

the convergence zone, producing vast quantities of granite.

Figure 4: Reconstruction of collisional tectonics during the Acadian orogeny. D1-D2 detail the development of a new basin as well as the uplift of Vermont due to continent collisions (from Doolan, 1996).

Mesozoic Era Extension• The breakup of Pangaea formed the present Atlantic Ocean and

resulted in extension throughout New England. • Extension reactivated faults formed during earlier periods. • Extension also resulted in magmatism in the Green Mountains,

producing lamprophyric dikes and small alkaline bodies. • Magmatism could be related to a mantle plume beneath the

North American plate or due to the rifting of the continental plate.

Pleistocene Glaciation• During the late Pleistocene, the

Laurentide Ice Sheet spread across New England. • The glacial till that separated the ice

from the underlying bedrock ground against the Green Mountains as it advanced. • The ice flowed obliquely across the

mountains from northwest to southeast, but changed direction and flowed from northeast to southwest as the ice sheet retreated. • Possibility of local post-Laurentide

mountain glaciation within the Green Mountains, though this is questioned.

Figure showing the strike of striations in the Green Mountains from Pleistocene Glaciation (Wright, 2013).

Recent Geomorphic Processes

• Erosional processes currently comprise the main geomorphologic influence on the Green Mountains. • The absence or presence of vegetation on the Green Mountains is

one of the most influential factors contributing to hillside erosion. • More extreme amounts of hydrologic activity due to climate change

has also increased hillside erosion. • The stability of the Green Mountain slopes is decreased by the

prominence of moisture-laden storms and freeze-thaw cycles.

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561-572. • Bierman, Paul, 1997: Postglacial Ponds and Alluvial Fans: Recorders of Holocene Landscape History. GSA Today, v. 7, no. 10, p. 1-8. • Coish, Raymond A., 2010: Magmatism in the Vermont Appalachians. Geological Society of America, v. 206, p. 91-110. • Conrad, Diane, and D. Vanacek, 1990: Welcome to Industrial Minerals of Vermont: 200 Years and Going Strong. Vermont Geological Survey, p. 1-2. • Davis, P. Thompson, 1999: Cirques of the Presidential Range, New Hampshire, and surrounding alpine areas in the northeastern United States. Géographie

physique et quaternaire, v. 53, p. 25-45. • De Souza, S., A. Tremblay, and G. Ruffet, 2014; Taconian orogenesis, sedimentation and magmatism in the southern Quebec–northern Vermont Appalachians:

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p. 11-12. • Wagner, Philip W, 1970: Pleistocene Mountain Glaciation, Northern Vermont. Geological Society of America Bulletin, v. 81, p. 2465-2469. • Wright, Stephen F., 2003: Glacial Geology of the Burlington and Colchester 7.5’ Quadrangles, Northern Vermont. Vermont Geological Survey, p. 1-12. • Wright, Stephen F., 2013: Laurentide Ice Sheet Flow across the Central Green Mountains, Vermont. Geological Society of America Abstracts with Programs, v.

45, p. 105. Images: http://www.findandgoseek.net/blog/2012/10/02/deals-and-steals-for-families-skiing-in-vermont-this-winter/http://dailyoffice.org/2012/10/20/morning-prayer-10-20-12-proper-23-ordinary-time/