Frenchman Bay Islands
Frenchman
(Above) View of the relief of Bald Rock metasiltstone.
Bald Rock
Bald Rock is a small island approximately 1 3/4 miles from the beach at College of the Atlantic. Barely more than an exposed ledge, the island rises roughly 30 feet out of the water and at high tide is about 150 feet across at its widest point. Despite being relatively near the mainland, soil on Bald Rock is scarce, and thus there are few plants capable of growing there. This gives the island its name and allows the intrepid geologist to examine the bedrock with ease.
Bar Harbor formation (Bhfm) makes up the majority of this remote shoal. This middle-Ordovician rock is noted for being layered and fissile, with strata consisting of silt to sand-sized particles. While this rock is already considered slightly metamorphosed, the fine-grained metasiltstone at Bald Rock has been further cooked due to a basalt dike that runs the length of the island. It is not unlikely that this contact metamorphism is why Bald Rock is here— the process of recooking metamorphosed sandstone further strengthened this particular part of the formation.The fissile properties of the Bar Harbor formation are readily noticed on this ledge, which steadily calves off into the sea in large, rectangular chunks. Through the middle runs a 1’ thick intrusion of an aphanitic, highly mafic rock (basalt) which has clearly affected the surrounding formation. Weathering processes have split the island into two halves, and the gap continues to widen. A small stone arch on the northern ledge owes its existence strictly to localized cooking and subsequent weathering by wind and sea.
The high-tide landing at Bald Rock with Mount Desert Island in the background. The island has been separated into two halves due to glacial activity and consistent erosion.
Bar Island is composed of two different types of bedrock. Along the southern edge of the island we find the Bar Harbor Formation (Bh Fm). This rock is a sedimentary sand/siltstone which formed 465 million years ago as sediments collected under the sea and were buried and lithified. The northern portion of Bar Island is composed of gabbro-diorite (Dgd), an intrusive igneous rock which formed as magma intruded into the Bar Harbor Formation and cooled to form a medium-fine grained crystalline rock. This rock composition and history is repeated in all of the Porcupine Islands in Frenchman’s Bay.
Bar Island Geologic History
Description of Rocks & Minerals
The Bar Harbor Formation is a meta-sedimentary rock which formed 465 million years ago. It is composed of layered sandstone, siltstone, and some volcanic ash. Layers of fine grained and well sorted, sub-rounded grains range in color from dark gray to cream and pink. Many pockets of crystals occur throughout the sample, including quartz, plagioclase feldspar, and muscovite. This rock formed when sand and silt accumulated on a marine platform and were buried and lithified over time. Nearby, active volcanoes in the region contributed ash to the Bar Harbor Formation. The BH Fm was then locally metamorphosed to a metasedimentary metasiltstone when large volcanoes of igneous rocks intruded the BH Fm.
Bar Harbor Formation outcrop on the southern edge of Bar Island.
Gabbro-Diorite (Dgd) is a mafic igneous rock is composed mostly of the minerals amphibole, plagioclase feldspar, and quartz. It is fine-grained and phaneritic in texture. It formed 420 million years ago, during a period of large intrusions on Mount Desert Island, this Dgd was intruded into the older SD BH. This rock seems to be slightly more resistant and therefore has formed islands in Frenchman’s Bay.