Geological History
About 20,000 years ago, continental glaciers covering North America began to melt and retreat northward. The glaciers melted due to changes in global climate changes caused by gradual changes in the earth's orbit and orientation to the sun. These glaciers had dramatic impacts on the natural landscape. At its peak 14,000 years ago, what is now the Connecticut River Valley was once a long-stretching glacial lake called Lake Hitchcock . The Ottauquechee River, which flows through Quechee Gorge, was one of the rivers that fed Lake Hitchcock. At their meeting point, the Ottauquechee River formed a delta with the lake.
Lake Hitchcock Map (U.S. National Park Service)
About 13,000 years ago, the landmasses holding back Lake Hitchcock (in what is now Connecticut) broke, draining the lake in various stages over roughly 1000 years. Quechee Gorge formed when Lake Hitchcock drained. The emptying of the lake broke down the Ottauquechee River Delta causing great amounts of water to flow through the river. . When glacial Lake Hitchcock drained, the Ottauquechee River cut quickly through delta sediment, establishing the present course of the Ottauquechee River.
The bedrock in this area is what's called Gile Mountain Formation, which is a metamorphic rock that dates back to the Devonian age (about 400 million years ago). While most of the 165-foot depths of the Quechee Gorge formed over the 1000 years that Lake Hitchcock drained, the Ottauquechee River still erodes the bedrock today as it flows through the gorge.
Fractures and Foliation
At the bottom of the gorge, there is a significant amount of bedrock that is visible. This bedrock shows both fractures and foliation. Fractures occur when rocks break in response to stress. When a rock is acted upon by pressure that is not the same in all directions, or by shear stress (forces acting to “smear” the rock), minerals can become elongated in the direction perpendicular to the main stress. The pattern of aligned crystals that results is called foliation which can be seen as repetitive layering in metamorphic rocks. Each layer can be as thin as a sheet of paper, or over a meter in thickness
The steeply east dipping foliation in the bedrock is visible at the bottom of the gorge. A younger set of fractures cuts across the folded bedrock. The dominant foliation in the folded, metamorphic rock is visible and dips east. The younger fractures cut across the foliation. Water flows along the foliation planes and along some of the fractures.
References
Information above was referenced from the following sources:
Activities
Go down to the bottom of the gorge where you can walk out on the bedrock (assuming there hasn’t been flooding and these rocks are accessible. Do not attempt if the water is too high). Send me an email with the following information.
1. Foliation – look for foliation in the rocks (A) The rocks are all roughly the same angle; what angle are they facing (B) How thick are the lines of the foliation (C) What direction does the foliation run from
2. Fractures – look for fractures in the rocks (A) How big are the fractures (B) Describe what you think was the cause of the fractures
3. How do you think the geology of these rocks will progress over time and why do you think that is the case?
4. Post a picture of yourself or a personal item at the bedrock