Maine: An Encyclopedia


Popham Beach

in Maine is heavily influenced by collisions with the Earth’s plates and by the impact of glaciers.


The geologic history recorded in Maine’s bedrock covers close to 1.5 billion years – approximately one-third of the total age of the Earth. Over this period a variety of geologic processes – sedimentation, deformation (folding and faulting), metamorphism, and igneous activity – produced the complex bedrock geology that we see today. Our understanding of Maine’s geology is constantly changing with new research.

Rock formations may be grouped by characteristics. Stratified rocks (layered rocks, including both sedimentary and volcanic rocks) are distinguished on the basis of their age and possible place of origin. Igneous plutons (rocks formed from molten magma, which cooled at depth) include rocks ranging in age from 500-65 million years, with the preponderance being 350-400 million years in age.

Basically, the geologic history recorded in Maine’s bedrock spans over 1 billion years. Several major cycles of silt and rock deposits, and collisions of the Earth’s plates are responsible for the bedrock geology that we observe today. The interaction of the Earth’s plates is responsible for the most spectacular features of Maine’s geology.


Maritime historian William Hutchinson Rowe had these insights on the importance of glaciers in forming Maine’s seafaring culture.  Noting John Greenleaf Whittier’s reference to “O hundred-harbored Maine!,” he writes:

A thousand would have hit nearer the truth. . . .

Then there are the islands.  They range along the coast almost like a breakwater or a barrier reef. Thus long stretches, such as from Portland to Mount Desert, have an inner passage where the waters are relatively quiet and there are harbors with depth and holding ground adequate for any craft. . . . The mainland, too, is a very nursery for ships, with deep coves for their cradles and clustering islands and high promontories to shelter them and break the force of the winds.

It was the glaciers of the Ice Age that first molded Maine into a habitation fit for men who would build, sail, and love wooden ships.

About 120 million years have passed between the formation of the youngest bedrock in Maine and the Pleistocene “ice age.” There is little evidence of the weathering and erosion of the land that occurred during this long interval, though remnants of deeply disintegrated bedrock that escaped later erosion by glacial ice can be seen in places.

Continental glaciers (like the ice sheet now covering Antarctica) probably extended across Maine several times during the Pleistocene Epoch, which lasted from about 1.5 million to 10,000 years ago. The slow-moving glacial ice changed the landscape as it scraped over previously existing mountains and valleys, transporting rock debris for miles. The sand, gravel, and other sediments that cover much of Maine are largely the product of glaciation. Some of these materials were deposited directly from glacial ice; others washed into the sea or accumulated in meltwater streams and lakes as the ice receded. Glaciation also disrupted earlier stream patterns and created the hundreds of ponds and lakes scattered across the state.

Most glacial activity in Maine involved the large continental ice sheets, but the erosional effects of alpine glaciers are clearly evident in two of the highest groups of mountains: Mt. Katahdin and neighboring peaks in Baxter State Park, and Sugarloaf Mountain and other nearby peaks in western Maine. Local glaciers eroded deep semi-circular bedrock basins (cirques) high on the flanks of these mountains. The basins on the east side of Mt. Katahdin are the best-known examples.

The oldest rock in Katahdin Woods and Waters National Monument, a light greenish-gray quartzite interlayered with slate from the early Cambrian period (over 500 million years ago), can be observed along the riverbank of the Penobscot East Branch for over 1,000 feet at the Grand Pitch (a river rapid).

Various formations in the area provide striking visual evidence of marine waters in Katahdin Woods and Waters during the geologic periods that immediately followed the Cambrian period. For example, Owen Brook limestone, an outcrop of calcareous bedrock west of the Penobscot East Branch containing fossil brachiopods, is of coral reef origin. Pillow lavas, such as those near the summit of Lunksoos Mountain, were produced by underwater eruptions. Haskell Rock, the 20-foot-tall pillar in the midst of a Penobscot East Branch rapid, is conglomerate bedrock that suggests a time of dynamic transition from volcanic islands to an ocean with underwater sedimentation. This conglomerate, deposited about 450 million years ago, contains volcanic and sedimentary stones of various sizes, and occurs in outcrops and boulders in several locations.*

The most recent glacial episode in Maine began about 25,000 years ago, when the Laurentide ice sheet overspread New England. During its peak, this ice sheet was centered over eastern Canada and flowed east to southeast across Maine. It became several thousand feet thick and covered the highest mountains. Rock debris was incorporated into the base of the glacier, causing the ice to scrape the bedrock surface over which it flowed. The grooves and fine scratches (striations) resulting from this process are often seen where bedrock has been freshly exposed. They are important in determining the directions of former ice movement. Erosion and sediment deposition by the ice sheet combined to give a streamlined shape to many hills, the long axes of which are parallel to the ice flow.

Climatic warming forced the Laurentide ice sheet to start receding as early as 21,000 years ago, soon after it reached its terminal position on Long Island. The ice margin also withdrew from the continental shelf east of Long Island and reached the present position of the Maine coast by 13,800 years ago. At this time the Earth’s crust was still depressed by the weight of the ice sheet. The sea flooded southern Maine as the glacier retreated to the northwest, extending far up the Kennebec and Penobscot valleys, and reaching present elevations of up to 420 feet in the central part of the state.

Great quantities of sediment washed out of the melting ice and into the sea. Sand and gravel accumulated as deltas and submarine fans where streams discharged along the ice front, while the finer silt and clay dispersed across the ocean floor. The shells of clams, mussels, and other invertebrates are found in the glacial-marine clay that blankets lowland areas of southern Maine. Radiocarbon dates on these fossils tell us that the marine submergence lasted until about 11,000 years ago, when it was terminated by uplift of the Earth’s crust as the weight of the ice sheet was removed.

By 12,000 years ago the glacier had shrunk to a local ice cap covering northern Maine. The last remnants of glacial ice probably were gone from Maine by 10,000 years ago. Large sand dunes accumulated in late-glacial time as winds picked up outwash sand and blew it up onto the east sides of river valleys, such as the Androscoggin and Saco valleys. The modern stream network was soon established, and organic deposits began to form in peat bogs, marshes, and swamps. Tundra vegetation bordering the ice sheet was replaced by changing forest communities as the climate warmed.

Geologic processes are by no means dormant today, however, since rivers continue to erode the land, and worldwide sea level is gradually rising against Maine’s coast. Low-intensity earthquakes and possible subsidence of the Earth’s crust also have been recorded during historic time.

From “The Geology of Maine.” Robert G. Marvinney and Woodrow B. Thompson. Natural Resources Information and Mapping Center, Maine Department of Conservation. Rowe citation added.

Additional Resources

Caldwell, D. W., 1972, The Geology of Baxter State Park and Mt. Katahdin. Maine Geological Survey, Bulletin 12, 57 p.

Jackson, C. T., 1837, First report on the geology of Maine. Smith & Robinson, Augusta 128 p.

Loiselle, M. C., and Thompson, W. B., 1987, The geology of Maine: Rocks and Minerals, v. 62, p. 386-392.

Maine Geological Survey. (accessed December 3, 2011)

*Obama, Barack. “Presidential Proclamation — Establishment of the Katahdin Woods and Waters National Monument.” Washington, DC. August 24, 2016. (accessed July 3, 2017)

Rowe, William Hutchinson. The Maritime History of Maine: Three Centuries of Ship Building and Seafaring. W.W. Norton. 1948. Reprinted: Gardiner, Me. Harpswell Press. 1989. pp. 13-14.

Whittier, John Greenleaf. “The Dead Ship of Harpswell.” (poem)

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This entry was last modified: July 05, 2017 12:12 AM

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