Mount Greylock - Geology and Ecosystem

Geology and Ecosystem

Mount Greylock and the neighboring Taconic Mountains are comprised predominately of Ordovician phyllite, a metamorphic rock, overlain on younger layers of metamorphized sedimentary rock, especially marble. Mount Greylock is the product of thrust faulting, a tectonic process by which older rock is thrust up and above younger rock during periods of intense mountain building. The younger, underlying marble bedrock layers have been quarried in the lower foothills of the mountain in nearby Adams and North Adams, Massachusetts. During the Pleistocene, 18,000 years ago, Mount Greylock and the surrounding region were covered by ice sheets up to 1-kilometer (0.62 mi) in thickness. Glaciation rounded and wore down the mountain, carving out U-shaped valleys and leaving glacial erratics such as the balanced rock on the west side of Greylock. The Hopper, a cirque, also located on the west side of Greylock, is the southernmost such glacial feature in New England.