Bar Island is one of several islands in Frenchman's Bay collectively named the Porcupine Islands. It stands out from the rest of the of the Porcupine Islands because of its naturally occuring sand bar that connects it to the shore of Bar Harbor, making it passable at low tide. When the bar is exposed, many locals and visitors make their way across to walk the paths and shorelines of the island, and get a nice view of the downtown area from the top of the hill. But views and scenery aren't the only exciting thing that Bar Island has to offer. It is rich with geologic history just waiting to be explored.
There are two different bedrocks that make up Bar Island, one being the Bar Habor Formation dating back ~465 million years, and the other a gabbro-diorite, ~420 million years old (Gilman and Chapman, 1988; Bruan and Bruan, 2012). These two bedrocks are best observed at the beach at the College of the Atlantic, and you can read about them here.
Bar Island has been shaped by tectonic, volcanic, and glacial activity. 465 milion years ago, the Bar Harbor marine sediments were deposited to form the Bar Harbor Formation (Gilman and Chapman, 1988; Bruan and Bruan, 2012). Sometime later down the line, tectonic plate movement caused the horizontal sedimentary rock to deform and tilt (Gilman and Chapman, 1988; Bruan and Bruan, 2012). These layers can be seen just north of where the bar meets the island, along the beach. See the minimap below for the location of the geological contact, the place where two different bedrocks touch.
A supervolcano that existed on the island ~420 million years ago formed the gabbro-diorite that makes up the majority of the island (Gilman and Chapman, 1988; Bruan and Bruan, 2012). The bedrock formed when magma intruded into the Bar Harbor Formation and cooled. There are other smaller intrusions on the island that are clearly visible, such as felsic granite dikes on the northern side of the island, felsic meaning that the rock formed from the melting of the earth's crust and is rich in quartz and felspar (Pictured Below).
Bar Island as a whole has been shaped by the movement of glacier. The Island is a roche moutonnée, a french word meaning "sheepback" named for its long upward slope until the peak of the of the hill, where at this point the slope becomes steep and rocky due to the movement of the glacier having plucked the rock into loose, broken boulders and angular chunks. Can you see the resemblance between the island and the diagram below?
If you plan on walking to the summit of the island, be sure to look at the cracked and deformed bedrock to see the evidence of the roche moutonnée! See the minimap to the right for the location of the summit.
On the northern side of the island along the shore sits a large granite boulder precariously placed in its position over time by glacial activity and other forces of nature. This kind of rock is called a glacial erratic, meaning it was carried by a glacier to its current resting place. See the minimap below for the erratic's location!
