Why Maine's coastline shifts from sandy to jagged rock

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The stark contrast between the curved, sandy beaches south of Portland and the indented, rocky coastline to the northeast is clear in this image captured by the OLI (Operational Land Imager) on Landsat 9 on August 31, 2025.

NASA Earth Observatory/Michala Garrison

The Wabanaki people have a deep well of creation myths explaining the rocky coastlines of the Bay of Fundy , Downeast Maine , and Acadia National Park . Many involve Glooscap —a magical figure said to have floated down the Bay of Fundy in a stone canoe , sculpting coastal features by scraping the vessel across the landscape and scattering enormous boulders during battles with primordial beavers, frogs, moose, whales, and other gigantic animals.

Fewer Indigenous creation myths survive to explain the origins of the sandy and marshy shorelines of southern Maine and the rocky, indented coasts of the state’s Midcoast region . But the sharp contrast between the sandy shoals and beaches south of Portland and the rocky shoreline of promontories, headlands , and narrow peninsulas to the east—visible in the Landsat image above—has long drawn the attention of coastal geologists, whose scientific explanations on its origins abound.

The coastal transition reflects both differences in the underlying bedrock and the distribution of sediment left behind by the last glacial maximum , coastal geologists say. Southern Maine has broad deposits of sand, much of it sourced from rivers. The sandy beaches of Saco Bay, for instance, home to Maine’s longest contiguous beach and the state’s largest saltmarsh , received sediment from the weathering and breakdown of the White Mountains , with material transported to the coast largely by the Saco River, explained Peter Slovinsky, a geologist with the Maine Geological Survey. Waves and tides reworked these soft sediments over time, sculpting them into the arch-shaped embayed beaches and sprawling salt marshes found around Saco Bay and the broader region.

While erosion-resistant granite juts from the sandy shorelines in southern Maine to form rocky headlands, metamorphic bedrock becomes the dominant surface feature east of Portland. There, whole ridges and valleys made of rock layers transformed by exposure to high pressures and temperatures define the landscape. During the last ice age, glaciers scoured and widened many of these coastal valleys, which later flooded as the Laurentide Ice Sheet melted and sea levels rose.

Around Casco Bay , these ridge-and-valley systems, combined with the drowning of the shoreline, produce the jagged, highly indented shoreline and many long, narrow islands seen today. “The tortured folds of these old landscapes also set up a sharp directional preference for erosion to exploit,” said Nicholas Whiteman, also a geologist with the Maine Geological Survey. “This led to the eye-catching difference in the orientation of the islands and necks that dominate Casco Bay compared with those to the northeast.”

The various forms that coastlines take fascinate geologists, but they also carry everyday implications for the economies of Maine’s coastal communities. While tourists flock to the sandy beaches of communities like Saco and Kennebunkport, the state’s iconic lobster fisheries are concentrated in Midcoast Maine. The crustaceans thrive in the cold waters of the region’s many rocky, protected inlets, turning communities such as Harpswell into leaders in lobster landings.

The state’s oyster farms are also concentrated in this region. Casco Bay and the Damariscotta Estuary, sheltered from winds and waves, offer waters that farmers can easily access without large boats. These waters provide a range of temperatures, salinities, and other characteristics that create numerous microclimates where oysters can grow quickly and take on a variety of tastes, known as merroir , explained Tom Kiffney, a researcher at the University of Maine. Kiffney is part of a team of researchers using Landsat and other satellite observations to predict oyster growth rates and help identify the most promising locations for new oyster farms in Maine based on water temperatures and quality.

NASA Earth Observatory image by Michala Garrison, using Landsat data from the U.S. Geological Survey . Story by Adam Voiland.

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August 31, 2025

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References & Resources

Kelley, J. (1987) An Inventory of Coastal Environments and Classification of Maine’s Glaciated Shoreline . Glaciated Coasts, 151-176.

Kiffney, T., et al . (2026) Using dynamic energy budget modeling and high-resolution satellite products to predict eastern oyster growth at a farm scale . Aquaculture , 612(1), 743133.

Leland, C. (1884) Algonquin Legends of New England: How Glooskap sailed through the great Cavern of Darkness . Accessed July 17, 2026.

Maine.gov (2026) Maine’s Coastal Marine Geology . Accessed July 17, 2026.

Maine Geological Survey (2002) Simplified Bedrock Geologic Map of Maine . Accessed July 17, 2026.

Maine Geological Survey (1999) The Variety of Maine’s Changing Shoreline . Accessed July 17, 2026.

Merroir (2026) What is Merroir ? Accessed July 17, 2026.

NASA (2026, January 15) NASA Data Helps Maine Oyster Farmers Choose Where to Grow . Accessed July 17, 2026.

NASA Earth Observatory (2017) Oyster Prospecting With Landsat 8 . Accessed July 17, 2026.

Snyder, J., et al . (2017) Oyster Aquaculture Site Selection Using Landsat 8-Derived Sea Surface Temperature, Turbidity, and Chlorophyll a . Frontiers in Marine Science , 4(190).

World History Encyclopedia (2025, February 6) Glooscap Tales . Accessed July 17, 2026.

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