The College of the Atlantic Beach
The beach at the College of the Atlantic is a great place to examine the two types of bedrock that make up Bar Island and the other Porcupine Islands. Both rocks, one sedimentary and one igneous, have their own outcrops on separate sides of the beach. A contact between the two rocks lies somewhere beneath the boulders and extends beneath the bay all the way to Bar Island and beyond.
(Left)
A closeup of the Bar Harbor formation.
(Left)
Outcrop scale of the Bar Harbor Formation. Student for scale.
On the northern side of the beach, you'll see the Bar Harbor Formation, a sedimentary rock with visible layers of silt and sandstone, as well as some layers of volcanic ash. These ancient marine sediments were deposited ~465 million years ago (Gilman and Chapman, 1988; Braun and Braun, 2012). The Bar Harbor Formation is slightly foliated and metamorphosed due to having been baked by the nearby Cadillac Mountain Granite when it intruded as magma that, at the time, reached extreme temperatures.
(Left)
Closeup of a Gabbro-diorite intrusion.
(Left)
Outcrop scale of a Gabbro-diorite intrusion. Same student for scale.
On the southern side of the beach you'll see gabbro-diorite, an igneous rock formed from magma that cooled below the surface of the earth. This rock also makes up most of Bar Island and the other Porcupine Islands, and crystallized ~420 million years ago (Gilman and Chapman, 1988; Bruan and Bruan, 2012). The texture of the rock is phaneritic, meaning the crystals of the rock are similar in size and visible to the naked eye. The gabbro-diorite intruded into the Bar Harbor Formation at around the same time the Cadillac Mountain Granite crystallized.