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Grindle Point, Islesboro
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.
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.
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.
Stewart, David B., Tucker, Robert D., Ayuso, Robert A., and Lux, Daniel R., 2001, Minimum age of the Neoproterozoic Seven Hundred Acre Island Formation and the tectonic setting of the Islesboro Formation, Islesboro block, Maine: Atlantic Geology, v. 37, p. 41-59.
Islesboro Formation listing in the U.S. Geological Survey database.
Maine Air Photo Viewer, Maine Office of Geographic Information Services.
Web site content by Henry N. Berry IV
Web site design by Susan S. Tolman
Originally published on the web as the December 2007 Site of the Month.
Last updated on April 12, 2012
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