The Geology of Acadia National Park, Maine

Acadia National Park is one of the most geologically remarkable places on the Atlantic coast, and the only national park on the eastern seaboard whose coastline was carved by Pleistocene continental glaciers. Its Mount Desert Island, the largest of the many islands off the Maine coast, holds Cadillac Mountain, the highest point on the U.S. Atlantic coast. The National Park Service's Geodiversity Atlas, drawn from its Geologic Resources Inventory, summarizes the park's geology for education, geoconservation and management.

Bedrock

Bedrock, glaciers and the sea together shaped Acadia. Its metamorphic, igneous and sedimentary rocks record major tectonic deformation, volcanic activity and the sediments that followed, making the park an excellent outdoor laboratory. Highlights include the once-molten rocks of the Cadillac Mountain intrusive complex, the granite of the Schoodic Peninsula, the volcanic episodes recorded in the Cranberry Island Series, the pink granite at the core of Mount Desert Island, the shatter zone bordering the Cadillac Mountain granite, diabase dikes cutting through the granites, and the joint sets that fractured the bedrock.

Glaciers

Continental glaciers covered even the highest peaks of Mount Desert Island and flowed into the Gulf of Maine, carving the ridges and valleys at the park's core. Rock debris in the ice polished the bedrock and cut striations, grooves and chatter marks in the granite. The ice shaped bedrock knobs called roches moutonnées and U-shaped valleys running to the sea. Meltwater carried sediment to the ice margin as outwash and deltas; moraines of till dammed the valleys; and boulders carried far from their source — glacial erratics — were left stranded on mountainsides.

About 21,000 years ago the ice sheet reached its greatest extent, stretching across the Gulf of Maine onto the continental shelf approximately 600 km (370 mi) beyond today's coast. So much water was locked in ice that sea level fell about 100 m (330 ft) below today's. Around Acadia the ice may have been 1.6 km (1 mi) thick; each acre of ice that thick weighs approximately 7 million tons.

A coast that sank and rose

Glacial ice pushes the land down about 30 m (100 ft) for every 91 m (300 ft) of ice. During the last glaciation, the Wisconsin, the weight pressed the coast down so far that parts of Mount Desert Island now above water were submerged, and the shoreline lay just south of Jordan Pond, approximately 70 m (230 ft) higher than today.

As the ice melted, the crust rebounded and relative sea level fell, exposing marine sediments on the island's slopes, and streams cut valleys into the newly exposed shelf. As rebound slowed and glaciers kept melting, sea level rose again to its present position, drowning valley mouths and creating New England's characteristic drowned shoreline: peaks became islands, ridges became headlands and peninsulas, and Georges Bank — islands at the height of glaciation — went under.

The shore today

• Tides and cliffs. A 3 to 4 m (10 to 12 ft) tidal range leaves tide pools full of sea life. Most of the coast is rocky cliff; since sea level rose after the ice age, little sand has built up, and most small pocket beaches hold more cobbles than sand.
• Sand Beach, between Great Head and Otter Cliff, is made of finely ground shell fragments (calcium carbonate) thousands of years old. It and a beach on Cranberry Island (outside the park) are the only Atlantic beaches north of Cape Hatteras made almost entirely of calcium carbonate sand.
• Sea caves. At least 12 sea caves, formed by differential erosion, lie along the coast; where one breaks through a headland it becomes a sea arch. Older caves from a higher sea level sit inland. All NPS caves are protected by the Federal Cave Resources Protection Act of 1988.
• Sea level is still rising, and nor'easter waves keep reshaping cliffs, caves and wave-cut platforms.

Acadia's rocky shoreline. NPS.

Fossils in Ice Age clay

After the ice retreated, most of coastal Maine, Acadia included, was briefly under the sea. The Presumpscot Formation (Presumpscot Clay, also called Leda Clay), laid down between 15,000 and 11,000 years ago as silt and clay settled in seawater while the land was pressed 100 to 120 m (330 to 395 ft) below sea level, is the region's only named Quaternary unit. Its fossils are largely marine: pollen, spores, logs and other plants; foraminifera, sponge spicules, bryozoans, bivalves, gastropods, the shell-dwelling polychaete worm Spirorbis, beetles, ants, barnacles, decapod crustaceans such as crabs, shrimp and lobsters, ostracodes and brittle stars; fish; and rare large mammals — walruses, whales and at least one mammoth. Walrus remains have come from Andrews Island, Addison Point and Gardiner, and mammoth bones from Scarborough, 140 km (87 miles) southwest of Isle au Haut. All NPS fossils are protected by the Paleontological Resources Preservation Act of 2009.

Hazards

• Slope failure. Rain falls every month; temperatures can drop below -18° C (0° F), but frequent thaws prevent deep snowpack, and ice storms are common in winter and early spring. Each spring, saturated soils, thawing, rain and storm waves cause rockfalls onto the Park Loop Road and slumping of coastal bluffs. Slumping marine clay has destabilized the bank of Hunters Brook and moved its course, and eroded, gullied clay slopes appear at the east end of the beach at Otter Cove.
• Earthquakes. Maine has a low but steady rate of earthquakes, generally under magnitude 4.8, and quakes near the park have set off landslides and damaged roads and trails.

The park's Geologic Resources Inventory geologic map. NPS.

Acadia belongs to the New England physiographic province. Its Geologic Resources Inventory maps and report, and a completed Soil Resources Inventory, are in the NPS DataStore.

Sources

Rewritten from the National Park Service article "NPS Geodiversity Atlas—Acadia National Park, Maine" (public domain), in hubnx's own words. The article puts the post-glacial low stand of the sea at 55 m (180 ft) below present in one section and 60 m (195 ft) in another; neither is given here. It spells Georges Bank "Georges Banks". The report cover it shows is not reproduced.