Geodiversity refers to the full variety of natural geologic (rocks, minerals, sediments, fossils, landforms, and physical processes) and soil resources and processes that occur in the park. A product of the Geologic Resources Inventory, the NPS Geodiversity Atlas delivers information in support of education, Geoconservation, and integrated management of living (biotic) and non-living (abiotic) components of the ecosystem.
Geologic Features and Processes
Arguably one of the most geologically remarkable sites along the Atlantic Coast, Acadia National Park represents the only National Park along the eastern seaboard with a coastline carved by Pleistocene continental glaciers. The park’s landscape captures the interplay between older episodes of tectonic deformation, Pleistocene glaciation, and more recent depositional and dynamic erosional processes of the ocean. The park’s Mount Desert Island, the largest of many islands located along the coast of Maine, contains Cadillac Mountain, the highest point on the U.S. Atlantic coast.
Bedrock, glaciers, and the sea combined to shape the exceptional landscape of Acadia National Park. The bedrock includes metamorphic, igneous, and sedimentary rocks that record significant tectonic deformation, volcanic activity and the resulting sedimentary response. Acadia National Park offers an excellent outdoor laboratory in which to examine the timing and geologic processes recorded in the bedrock. Excellent examples include the emplacement of previously molten rocks as part of the Cadillac Mountain intrusive complex on Mount Desert Island, the granite on the Schoodic Peninsula, and the volcanic episodes recorded in the Cranberry Island Series. In addition to the pink granite that forms the core of Mount Desert Island, other spectacular features include the shatter zone that borders the Cadillac Mountain granite, diabase dikes that cut the granitic units, and joint sets that fractured the bedrock.
Beaches and Coastal Landforms
The rocky shoreline wraps around the irregular border of Acadia National Park. Waves and currents continually modify the park’s shoreline. The 3 to 4 m (10 to 12 ft) tidal range leaves behind tide pools inhabited by diverse and abundant sea life.
Most of the coastline of Acadia National Park consists of rocky cliffs. Since sea level rose following the Pleistocene ice age, sand accumulation along the coast of Acadia National Park has been minor. Sandy beaches are relegated to small pockets of sand in coves and a few sandbars in protected areas. Most of the pocket beaches contain more cobbles than sand.
Sand Beach, sheltered between Great Head and Otter Cliff, is exceptional because its “sand” is composed of finely ground, sand-sized shell fragments (calcium carbonate). Deposited by waves, these shell fragments are thousands of years old. Sand Beach and a beach on Cranberry Island (not on park property) hold the distinction of being the only beaches along the Atlantic coastline north of Cape Hatteras, North Carolina, composed almost entirely of calcium carbonate sand.
Sea level continues to rise along the Atlantic Coast, and the Atlantic surf relentlessly pounds the Acadia shoreline. Periodically, waves generated by violent nor’easters unleash an enormous amount of energy against the parks’ cliffs and headlands. The constant hydraulic action creates and modifies the sea cliffs, sea caves, wave-cut platforms, and other coastal features that enrich the splendor of Acadia National Park.
The glacially-carved ridges and valleys of Mount Desert Island form the core of the park. Massive continental glaciers that covered the highest peaks of Mount Desert Island and flowed into the Gulf of Maine created both depositional and erosional features of the landscape. Abrasion by debris in the glacial ice polished the bedrock and carved striations, grooves, and chatter marks in the granite. The grinding force of the glaciers sculpted bedrock knobs known as roches moutonnées and carved characteristic U-shaped valleys that extended to the sea. Meltwater channels within the glaciers transported sediment that was deposited at the glacial margin in fluvial outwash and delta deposits. Debris dumped at the front of melting glaciers became thick moraines of glacial till that dammed the U-shaped valleys. As the glaciers melted, boulders that had been transported far from their place of origin, known as glacial erratics, were left stranded on mountain sides.
Acadia showcases a wide variety of glacial landforms today, including:
- Glacial Polish, Striations, Grooves, and Chatter Marks
- Glacial-derived Cliffs
- Roches Moutonnées
- U-shaped Valleys
- Meltwater Channels
- Glacial Till
- Terminal Moraines
- Glacial Erratics
- Glacial Outwash Deposits and Deltas
The surficial deposits in Acadia National Park record a remarkable interaction of glacial activity and sea level fluctuations. Glacial till was deposited directly by glaciers and consists of an unsorted collection of clay, silt, sand, pebbles, cobbles, and boulders. As the glaciers melted, ridges of mixed rock debris (ranging in size from clay to boulders) known as glacial moraines were left along the glaciers’ margins. Meltwater streams flowing from melting glacial ice deposited sand and gravel in outwash deposits in front of the glacier or in deltas building along the coast (Lowell and Borns 1988).
About 21,000 years ago, the continental ice sheet reached its maximum extent. The ice sheet flowed across the Gulf of Maine and extended onto the continental shelf approximately 600 km (370 mi) from the present Maine coastline. Because so much water was trapped in the continental ice sheets, sea level fell about 100 m (330 ft) below its current level (Gilman et al. 1988; Kiver and Harris 1999).
In the Acadia region, ice may have been 1.6 km (1 mi) thick. Each acre of ice 1.6-km (1-mi) thick weighs approximately 7 million tons (Kiver and Harris 1999).
Continental glaciation displaces the upper mantle so that the land surface subsides about 30 m (100 ft) for every 91 m (300 ft) of ice. During the last period of glacial activity, known as the Wisconsin glaciation, the mass of the ice depressed the coastline (isostatic depression) so that areas on Mount Desert Island that are now above sea level were submerged. At that time, the shoreline on Mount Desert Island lay just south of Jordan Pond, which was approximately 70 m (230 ft) higher than it is today (Gilman et al. 1988).
Glacial melting resulted in a complex interplay between crustal rebound and sea level change. Unburdened from the mass of ice, Maine’s crust began to rise. With crustal rebound (isostatic uplift), relative sea level fell. Submerged sediments that had been deposited in marine environments became exposed on the slopes of Mount Desert Island (Lowell and Borns 1988). Sea level lowered to approximately 55 m (180 ft) below its present position, and streams cut valleys into previously submerged coastal areas and continental shelf deposits (Barnhardt et al. 1995).
As crustal rebound slowed, glaciers continued to melt and sea level rose to its present position. Rising sea level drowned the lower ends of valleys and produced the submerged or drowned shoreline typical of New England. With rising sea level, peaks have become islands, and once rocky ridges have been transformed into headlands and peninsulas. The famous offshore fishing ground of Georges Banks, exposed as islands during the height of glaciation, became submerged during this time.
Cave and Karst
Sea caves found along the current coast are affected by wave action which causes natural changes to the caves. There are at least 12 known sea caves currently located in several coastal areas of Acadia NP. These caves were all formed by differential erosion. In some places, when sea caves enlarged enough, they break through a headland to form a sea arch.
Older sea caves from a higher sea stand are found inland and provide a more stable environment inside the cave.
All NPS cave resources are protected under the the Federal Cave Resources Protection Act of 1988 (FCRPA)(16 U.S.C. § 4301 et seq.).
Most of coastal Maine was briefly submerged after the last glacial retreat, with the glaciomarine Presumpscot Formation representing this period. Acadia is no exception to this, with much of its acreage under water at the time (Dorion et al. 2001). The Presumpscot Formation (or Presumpscot Clay; also called the Leda Clay) was deposited between 15,000 and 11,000 years ago (Richards and Belknap 2003). Present within Acadia, this formation is the only named Quaternary unit of the coastal region. It formed from silt and clay deposited in seawater during and after the glacial retreat, while isostatic loading locally depressed the land 100 to 120 m (330 to 395 ft) below sea level (Nielson 2006). Post-glacial rebound exposed the present near shore to 60 m (195 ft) below present, and later global sea-level rise flooded the shelf to its present configuration (Belknap et al. 1987; Barnhardt et al. 1997).
The Presumpscot Clay has yielded a diverse, largely marine fossil assemblage. Plant material includes pollen, spores (Knox 1956), logs (Thompson et al. 2008), and other plant macrofossils (R. Anderson et al. 1982). Invertebrates include foraminifera (protists that form “shells”), sponge spicules (Knox 1956), bryozoans (“moss animals”), bivalves, gastropods, the shell-dwelling polychaete worm (bristleworm) Spirorbis (Brown and Branson 1932), beetles, ants (Kilian and Nelson 2008), barnacles, decapod crustaceans (crabs, shrimp, lobsters, and allies), ostracodes (seed shrimp), and ophiuroids (brittle stars) (Brown and Branson 1932). There have also been some vertebrate finds, including fish (Brown and Branson 1932) and a few rare large mammals, such as walruses, whales (Caldwell 1998), and at least one mammoth (Hoyle et al. 2004). Walrus remains have been reported from localities including Andrews Island, 31 km (19 miles) west of the Isle au Haut unit, Addison Point, 37 km (23 miles) northeast of the Schoodic Peninsula unit, and Gardiner, 92 km (57 miles) west-northwest of Isle au Haut (Hay 1923; all of these are not necessarily from the Presumpscot Formation). The mammoth bones were found at Scarborough, 140 km (87 miles) southwest of Isle au Haut (Hoyle et al. 2004).
All NPS fossil resources are protected under the Paleontological Resources Preservation Act of 2009 (Public Law 111-11, Title VI, Subtitle D; 16 U.S.C. §§ 470aaa - 470aaa-11).
Natural geologic processes continue to shape the park on time scales ranging from seconds to years. Natural materials and processes can create geologic hazards and associated risks. Be cautious and alert to geohazards that may be present in the park, including:
Mass Wasting and Slope Failure
Rain falls every month in Acadia National Park. Although temperatures can fall to below -18° C (0° F), frequent thawing periods prevent large, long-term snow accumulations. Ice storms are common in winter and early spring (National Park Service 2006b). Every spring, saturated soils, thawing, precipitation, and storm waves lead to rockfalls that impact the Park Loop Road on Mount Desert Island and slumping along coastal bluffs.
Mass wasting of marine clay (deposited at a time when sea level was much higher) occurs along Hunters Brook, which drains into the cove protected by Hunters Head on Mount Desert Island. Slumping of the greenish- gray clay has destabilized the stream bank and changed the course of the stream. Exposures of eroded and gullied slopes, characteristic of this marine clay, may also be found on the eastern end of the beach at Otter Cove. Potential mass wasting and slope failure may be an issue wherever this marine clay is exposed on Mount Desert Island (Lowell and Borns 1988).
Earthquakes with epicenters close to the park have triggered landslides and damaged roads and trails. Earthquakes occur at a low but steady rate in Maine and generally have magnitudes less than 4.8 on the Richter scale. While smaller than west-coast earthquakes, they do increase the potential for rockfall and mass wasting.
Geology Field Notes
Students and teachers of college-level (or AP) introductory geology or earth science teaching courses will find that each park's Geologic Resource Inventory report includes the Geologic History, Geologic Setting, and Geologic Features & Processes for the park which provides a useful summary of their overall geologic story. See Maps and Reports, below.
Acadia National Park is a part of the New England Physiographic Province and shares its geologic history and some characteristic geologic formations with a region that extends well beyond park boundaries.
- Scoping summaries are records of scoping meetings where NPS staff and local geologists determined the park’s geologic mapping plan and what content should be included in the report.
- Digital geologic maps include files for viewing in GIS software, a guide to using the data, and a document with ancillary map information. Newer products also include data viewable in Google Earth and online map services.
- Reports use the maps to discuss the park’s setting and significance, notable geologic features and processes, geologic resource management issues, and geologic history.
- Posters are a static view of the GIS data in PDF format. Newer posters include aerial imagery or shaded relief and other park information. They are also included with the reports.
- Projects list basic information about the program and all products available for a park.
Related ArticlesAcadia National Park
National Park Service Geodiversity AtlasThe servicewide Geodiversity Atlas provides information on geoheritage and geodiversity resources and values within the National Park System. This information supports science-based geoconservation and interpretation in the NPS, as well as STEM education in schools, museums, and field camps. The NPS Geologic Resources Division and many parks work with National and International geoconservation communities to ensure that NPS abiotic resources are managed using the highest standards and best practices available.
For more information on the NPS Geodiversity Atlas, contact us.