Great Head
Great Head is a wonderful cliff trail northwest of Sand Beach that offers fantastic ocean views. The rocks along the trail offer insight into the deep Geologic history of the area. The stony beach cliffs that make up the beach at this site clearly display the Shatter Zone of MDI- evidence of an ancient super volcano. There are two parking lots within walking distance of the site: one off of Park Loop Road that rests above Sand Beach (that requires a toll), and another off of Schooner Head Road leading out of Bar Harbor (a good alternative if the Park Loop road is closed or crowded!).
Shatter Zone
Around 420 million years ago, a large magma chamber formed beneath what is now MDI. (Gilman, 2012) When it erupted, the explosiveness of the magma chamber broke up the surrounding rock. When the magma cooled, it formed chunks, called clasts, of the surrounding rock inside of the igneous rock that it formed. This is called the Shatter Zone (Marvinney, 2010), and it outlines the caldera of the super volcano, which is a crater left when the volcano erupted and collapsed in on its magma chamber (Marshak, 2012). At Great Head, it consists of clasts of the Bar Harbor Formation set into a light gray granite that contains quarts and hornblend minerals. The Shatter Zone can be seen in several locations on the island’s edge (Gilman, 2012), suggesting that the volcano’s magma chamber encompassed most of the island.
(Above) The Shatter Zone facing west. The dark patches are clasts of the Bar Harbor Formation.
Bar Harbor Formation
The clasts in the Shatter Zone are from the Bar Harbor Formation, which is a sedimentary rock that is also seen in an outcrop, separate from the Shatter Zone, at the western edge of the beach. A regular outcrop of it can be seen near the entrance to the area. Because it is included in the Shatter Zone, it formed earlier than the volcano, about 465 million years ago (Gilman, 2012). The state of Maine was formed by the collision of many different land masses as the tectonic plates of the earth moved, and this area was once across the ocean. The formation consists mainly of siltstone, which forms when sediments settle at the bottom of an ocean, harden, and are covered by more sediments (Marshak, 2012). This means that this rock formed in an ocean that once existed between continents, but closed up as the old continent collided with what is now Maine.
(Above) An outcrop of the Bar Harbor Formation, facing west, stands out darker at the ocean's edge.
Bedrock Fractures
Most of the rocks here have cracks in them, known as fractures, which run in sets of at least 3 directions. In fact, the bedrock on most of the island have fractures running through them that are oriented in similar directions. These fractures also line up nicely with many of the landscape features on the island, such as the orientation of this beach. These fractures formed when the earth’s crust shifted long ago, and created planes of weakness (Slocum, 1978). Things that further affected the landscape, such as the super volcano and glaciers, would have followed them because they were the easiest points to move along.
Above: Fractures through the Great Head Bedrock.
References Cited
Gilman, Richard A. and Chapman, Carleton A., Bedrock Geology of Mount Desert Island: A Visitor’s Guide to the Geology of Acadia National Park: Maine Geological Survey, 1988: Revised by Braun, Duane and Ruth, 2012.
Marshak, Stephen, Earth: Portrait of a Planet: New York, London, W. W. Norton & Company, Inc. 2012.
Marvinney, Robert, Earth Shattering Events at Mount Desert Island!: Maine Geological Survey, 2010.
Slocum, Robert D., 1978, Friction Cracks as Directional Indicators of Glacial Flow on Mt. Desert Island, Maine: Ohio State University.