Grindle Point, Islesboro

Lighthouse at Grindle Point, Islesboro
Click on photo to enlarge

Introduction

air photo of the Grindle Point coastal ledges

One of the many jewels along the Maine coast is the island of Islesboro, in northern Penobscot Bay. There are about 600 year-round residents in the town of Islesboro, which includes the big island and several smaller neighboring islands. Primarily a residential town, summer visitors will find enjoyable activities for day trips such as bicycling and kayaking. The public ferry from Lincolnville lands on the western side of the island at Grindle Point. The topic of this web site is the geology of the coastal ledges (Figure 1) next to the ferry parking lot, just north of the Grindle Point Lighthouse and Sailor's Memorial Museum.

Islesboro Geology

Islesboro is distinctive among the larger islands of Penobscot Bay in being long and narrow, over 11 miles north-south, and only 2½ miles across at its widest. This shape reflects its bedrock, which is dominated by sandstone, slate, and limestone - layered rocks that are aligned predominantly north-northeast. Nearby islands such as Mount Desert Island, Swans Island, Deer Isle, and Vinalhaven, which are more nearly circular in outline, are underlain primarily by masses of granite.

The Islesboro Formation (Smith, Bastin, and Brown, 1907 - see references) comprises the bedrock of most of the island, and is preserved in very few places outside the island. It formed originally as a sequence of sedimentary rocks that later was changed by heat and pressure into the metamorphic rocks we see today. The rocks at Grindle Point contain thin layers that formed as the sediments were being deposited. The metamorphic events caused the rocks to be deformed, so that the layers are now folded into curved shapes, and disrupted by faults. It is the basic rock structure that controls the northeast trend of the island. The age of the original sediments is probably Cambrian(?) or late Precambrian(?), over 500 million years old. The age of metamorphism and faulting is not well known, but is probably of Silurian age (Stewart and Tucker, 1998; Stewart and others, 2001 - see references). Thin, dark green dikes of igneous rock cut the Islesboro Formation at many places.

Special Features of the Rocks at Grindle Point

The readily accessible ledges at Grindle Point show many representative features of the geology of Islesboro. Click on these photos to learn the significance of some things you can see at Grindle Point.

Figure 6
Fault Breccia

Figure 7
Glacial Grooves
and Striations

Slide Show of Grindle Point Geology

Now that you have learned what to look for, take a photo tour of the geology at Grindle Point by viewing our slide show. Please be patient as Java applets and associated images are downloaded to your computer.

For More Information, See These References

Smith, George Otis, Bastin, Edson S., and Brown, C. W., 1907, Description of the Penobscot Bay quadrangle, Maine: U. S. Geological Survey, Geologic Atlas, Folio 149, 14 p. (map scale 1:125,000).

Stewart, David B., 1974, Precambrian rocks of Seven Hundred Acre Island and development of cleavage in the Islesboro Formation: in Osberg, Philip H. (editor), Guidebook for field trips in east-central and north-central Maine: New England Intercollegiate Geological Conference, 66th Annual Meeting, Orono, Maine, p. 86-98.

Stewart, David, B., and Tucker, Robert D., 1998, Geology of northern Penobscot Bay, Maine: U.S. Geological Survey, Miscellaneous Investigations Map I-2551, scale 1:62,500, two sheets.