Yarmouth Island Geology Tour
By Alicia Pulsifer Heyburn
On a clear morning on the last day of September, I paddled my kayak to Bethel Point to meet Bates College students in Dyk Eusden’s field geology class. They had recently climbed Mt. Washington to learn about the ancient Appalachian Mountains, traveled to Ripogenus Gorge and will visit Vinalhaven, Acadia and a tourmaline quarry in Oxford Hills; but today they were paddling Quahog Bay to observe the Hen Cove Anticline, where its distinct metamorphic layers are exposed on Yarmouth Island.
Since 1901, the Pulsifer and Morton families have owned Yarmouth Island, and in 1990 they conserved it with Harpswell Heritage Land Trust. As a member of the Pulsifer family, 100+ years of ownership used to seem like a long time, until I compared our human history to the geologic timescale of the island.
The Hen Cove anticline is identified in the State of Maine’s Register of Critical Areas (now incorporated into the Maine Natural Areas Program), and is specifically mentioned in the island’s conservation easement. For many years Dr. Eusden has made arrangements with us to bring his students to specific points along the shoreline where they can use clues in the composition and orientation of the rock outcroppings to piece together events that happened over 400 million years ago.
The three distinct layers exposed in the north northeast-trending Hen Cove anticline are part of the East Harpswell Group, mapped by noted Bowdoin College geologist Arthur Hussey in the 1970s. They are, from youngest to oldest, the Bethel Point Formation, the Yarmouth Island Gneiss and the Yarmouth Island Amphibolite.
The first stop of our paddle was along the eastern shore of the island to observe the middle layer, the gneiss, at the peak of the anticline’s fold before it dips back under the water toward West Cundy’s Point. A distinct arch was evident in the folds of the rock. Gneiss is a grey rock characterized by its layered appearance, and according to Hussey, the origin of this formation was volcanic ash laid down on the ocean floor about 445 million years ago at the bridge of the Silurian and Ordovician eras. It was then altered through heat and pressure during metamorphism, almost 45 million years later during the Acadian orogeny; a significant geologic episode caused by collision of an ancient land mass with North America. Most rocks in Maine were affected while the land buckled, pushing up the ancestral Northern Appalachian Mountains (including Mt. Washington).
We scrambled over seaweed and back into our kayaks to paddle to the southeastern tip of the island where Yarmouth Island Amphibolite, the oldest layer the of the East Harpswell Formation, is exposed. Over many childhood summers I appreciated this sparkling and colorfully veined outcrop as my cousins and I walked the shore barefoot, but I never knew that the sparkle was from white quartz dehydrated during metamorphosis, the thin black slivers were amphibole and biotite (a mica), and the cinnamon brown swirls were garnet heavy with manganese and rimmed with pistachio green opidote. The Paleo environment of my summer playground was a rioting eruption of basalt, with hydrothermal vents depositing minerals that were all metamorphosed when continents collided.
The tide was falling and the surf increasing, so we returned to our boats to continue in a clockwise pattern around the island, making our final stop on the western shore to observe the youngest rock of the East Harpswell Group, the Bethel Point Formation. The shore was primarily a rough, rusty red rock, studded with quartz, and a small pale grey outcrop weathered to smooth lumps and knobs like a Henry Moore sculpture. The grey stone is marble, which surprised me, but explained why it reminded me of a sculpture. Marble is metamorphosed limestone, perhaps an ancient coral reef or shell laden seafloor when the land that now comprises Yarmouth Island was about 10 degrees south of the equator in a tropical sea. When the students released a few drops of acid onto the rock it fizzed, indicating calcium carbonate. This is the same reaction that is eroding bridges, buildings and artwork exposed to acid rain.
I knew the site as “Caroline’s Beach,” so named by my daughter about a decade ago when she liked to go there to collect rocks. She was on to something, because the red is garnet, and Dr. Eusden said this is one of the widest bands of garnet that he has seen anywhere on the planet.
An iron rich band was likely deposited by a hydrothermal vent in a sea floor setting, and during metamorphosis turned to garnet. In some places garnet is of gemstone quality, but more frequently it is used as an industrial abrasive, such as on sandpaper. This site yields no gems, but is still interesting. One of the students asked about some orange splotches on the rocks and Eusden responded, “That is a subject for an entirely different class, it is a lichen. But I will say, that this orange lichen likes calcium, so that is a hint as to the type of rock it is on.”
Over the course of two hours we traveled across several hundred million years of geologic time, and my summer haven was transformed into a mesmerizing science classroom. The students returned to Bethel Point and then to campus to write papers on their observations and I went ashore to close our cabin for the winter.
Alicia Pulsifer Heyburn is a 5th generation owner of Yarmouth Island and a trustee of the Harpswell Heritage Land Trust
Please note that many privately-owned conserved lands do not provide public access. Click here for a list of trails and preserves open to the public, such as the Giant’s Stairs, a fascinating geologic formation on Bailey Island.