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Those potholes are not glacial...

photo of potholes

Text extracted with permission from Greenfield Community College Prof. Richard Little's book entitled Dinosaurs, Dunes, and Drifting Continents: The Geohistory of the Connecticut Valley, 2003 (available at this site: www.earthview.pair.com)









Almost 600 million years ago, our area was south of the equator and under a warm, shallow sea that also covered much of the United States. One hundred million years or so later, exploding volcanoes caused this ocean to begin disappearing.

A subduction zone developed several hundred miles eastward of the continental margin, creating a magnificent chain of tall strato-type volcanoes. Erosion of the volcanoes flooded the surrounding sea floor with layers of sand and mud, plus there was ash from the explosive eruptions. Coral built beautiful fringing reefs around the islands 50 million years before sharks or other fish swam the oceans.

This tropical paradise, known as the Bronson Hill Plate, is the main ancestral bedrock underlying the Connecticut Valley. Crustal pieces such as this Bronson Hill Plate can drift perhaps thousands of miles across oceans before they finally crash into a continental margin. They bring bedrock with a whole different history to merge with the rock of the "host" continent. These imported "terranes" are called "exotic" since they are so different from surrounding rock.

It is possible that a small plate with subduction zone volcanoes (an exotic terrane known as the Shelburne Falls Arc), collided with Laurentia (the continental plate carrying modern-day North America) before Bronson Hill arrived. Rock in the plate's volcanic and magma chamber then metamorphosed and is now exposed in domes.

The Shelburne Falls dome is the best example and gives its name to this terrane. The former magma chamber granite was extensively metamorphosed and as well as the whole region has been uplifted and eroded by pre- and post-glacial river action. During the retreat of the last glacier, Lake Hitchcock's shoreline was nearby, and the Deerfield River covered the future site of the Potholes with sand and gravel as it flowed into the lake.

The Potholes actually formed beginning about 14,000 years ago, after Lake Hitchcock had drained. The Deerfield River eroded onto this beautiful gneiss rock and drilled the famous potholes so well exposed in the bed of the River next to downtown Shelburne Falls.

Subduction zones indicate crustal convergence. The Shelburne Falls Arc and / or Bronson Hill, with abundant volcanoes and reefs , collided with the eastern margin of Laurentia, forming the complexly faulted and folded Taconic Mountains. Western New England's bedrock is composed of a complicated mix of both deep and shallow ocean sediment, plus lava (basalt) ocean crust and granite from subduction zone magma chambers. All are now compressed -folded, faulted and metamorphosed, into a complicated mix of the mountainous bedrock exposed for view for Berkshires travelers. See this fun site from PBS to learn more about subduction zones (contains animation- need Flash). graphic from PBS site

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Updated 6/11/07 by MF Walk DRWA HOME