GEOLOGY OF THE SHELBURNE FALLS POTHOLES By Richard D. Little, Prof. Emeritus Greenfield Community College www.earthview.pair.com RDLittle2000@aol.com

  The Shelburne Falls potholes reveal a very dramatic and interesting geologic story involving volcanoes, colliding continents, mountains as high as the Himalayas, and lots and lots of erosion. The potholes are the final event to finish this marvelous history. Let's take a closer look.

  The bedrock of the potholes is one of the world's most beautiful rock types. It is the metamorphic rock gneiss, pronounced "nice." This rock has undergone high temperatures and pressures due to the collision of tectonic plates that eventually created the supercontinent of Pangea. These collisions began in early Paleozoic time (Ordovician Period), about 480 million years ago. At this time a series of volcanic islands, similar to today's Japan or Indonesia, named "the Shelburne Falls Arc," composed the western New England landscape. These Shelburne Falls Arc volcanoes were a landmass several hundred miles offshore and not yet part of North America. The motion of plate tectonics pushed them into North America and added this land to our continent.

  To understand the origin of the potholes' rock, one must go underneath the Shelburne Falls Arc volcanoes. Beneath all volcanoes are magma chambers. These massive molten rock reservoirs slowly cool to become the igneous rock granite. Granite, which is usually a gray speckled rock, was transformed into beautifully swirling banded gneiss due to the heat and pressure from the collision of the tectonic plates.

  The merging of the Shelburne Falls Arc with North America created the Taconic Mountains, the first phase of a complicated series of mountain building events that are part of the eastern US Appalachian Mountains. The rocks we see along the Deerfield River today, were formed in the middle of these old mountains, about 10 miles deep down in the Earth's crust. Today, after 400 million years of erosion, we can now see the rock from deep in the Earth and appreciate the tremendous geologic forces that make mountains. Imagine the heat and pressure needed to produce the folded bands of light and dark minerals in the gneiss!

  The famous "Glacial Potholes" are actually not glacial at all. Potholes are formed by swirling river-bed stones that drill into the bedrock. They are common in all rivers that flow in rock channels. The "Glacial Potholes" began to form after the last glacier age when the Deerfield River first started to flow over these rocks, about 14,000 years ago. We can date the formation of these river-eroded features thanks to the great glacial lake, Lake Hitchcock, that filled the Connecticut Valley and also extended into the lower Deerfield Valley. While Shelburne Falls was not under Lake Hitchcock, it was under the sediments of the Deerfield River that built a delta into the lake. Lake Hitchcock drained by 14,000 years ago. The Deerfield River was then able to cut downward into its delta sediments. During this erosive process, which continues today, the river found itself on top of the gneiss bedrock and could start eroding holes in the hard gneiss.

  The largest "pothole" is actually a "plunge pool." While potholes have level rims around the hole drilled by the rotating river stones, plunge pools are created at the base of a waterfall. Waterfalls always have a high rim that the water falls over. The plunge pool is eroded by the impact of the falling water tumbling the river stones. Since there is more energy in a plunge pool, these features are always much larger than potholes. The Deerfield River channel exposes both a plunge pool and potholes. It is a wonderful place to understand and enjoy the beauty and power of nature.

  For more information:
Richard D. Little's "Dinosaurs, Dunes, and Drifting Continents: The Geology of the Connecticut River Valley", 3rd edition, 2003, 176 p., Earth View LLC, Easthampton, MA. www.earthview.pair.com
The Office of the Massachusetts State Geologist: www.geo.umass.edu/stategeologist