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Rocky coast at Acadia National Park
Acadia National Park, Maine

Bedrock Geology.

The geology of the Mount Desert Island region has been extensively studied since the late 1800s. The larger geologic units were first mapped by Crosby (1891), Davis (1881), Shaler (1889), Frazer (1906), Bascom (1919), and Raisz (1929). Several early geologic maps were prepared (Shaler, 1889; Keith, 1933; Perkins, 1933). Chapman (1970) and Gilman and Chapman (1988) updated the work of the early geologists.

Granite underlies most of Acadia, including much of Mount Desert Island, Isle au Haut and all of the park on Schoodic Peninsula (Gilman and Chapman, 1988; Chapman, 1970). This resistant bedrock makes up the high elevations and steep valleys that give the park its rugged character. Granites at Acadia vary slightly in texture, color, percentages of accessory minerals and chemical composition (Carl et al., 1984), and a number of sub-types have been described and mapped (Gilman et al., 1988). The granite underlying most of Mount Desert Island is a pink coarse-grained hornblende granite that contains minor biotite (the Cadillac Mountain Granite), while the other granites, including those at Schoodic and Isle au Haut are mostly fine-grained biotite granites (Gilman and Chapman, 1988).

Gabbro-diorite bedrock borders the granites on Mount Desert Island and Isle au Haut. A variety of older stratified rocks make up the margins of these islands. On Mount Desert Island, these include a schist, a siltstone/sandstone formation, and a volcanic series of tuff, felsite and interbedded volcanic and sedimentary rocks. In addition, a shatter zone exists at the contact between Mount Desert Island coarse-grained granite and the older stratified rocks of the Bar Harbor Formation (Gilman, et al., 1988). On Isle au Haut, most of the park shoreline is composed of volcanic rocks.

Chemically-resistant granites and rapid runoff result in surface waters with low alkalinity and low nutrient concentrations. Marine aerosols (Na and Cl) are the dominant ions in most surface waters and contribute more than half of the conductivity of Acadia waters. The mean conductivity of Acadia lakes is 44 µS/cm. Without contributions from Na and Cl, the typical lake conductance would be in the range of 15 to 20 µS/cm, compared to the mean value for all Maine lakes of 36 µS/cm. Upland streams that drain granitic terrain are typically even more dilute than lakes, whereas the valley streams have higher alkalinity and higher ionic strength from weathering and ion exchange reactions in thicker glacial and glaciomarine deposits.

Topography and Glacial Geology. The topography of Acadia consists of a series of barren ridges separated by glacially deepened U-shaped valleys. Mountains within the park rise to heights of 464 m (1,530 ft). The ridges and valleys trend north-south, as modified by glacial scouring. Many park landforms have a distinctly asymmetric profile, displaying gentle slopes to the north and northwest and steeper slopes to the south and southeast. This landform is the textbook example of the roches moutonnees geomorphology that results from glacial action on resistant bedrock (Flint, 1971).

The U-shaped valleys at Acadia are probably pre-existing drainages, which were considerably deepened and modified by Wisconsinan glacial ice that is inferred to have been up to one km thick over Mount Desert Island 14,000 to 30,000 years ago. Some lakes within the valleys were formed during deglaciation as glacial materials were deposited at the ends of the valleys, creating natural dams (Gilman et al., 1988). Acadia's glacial legacy is most evident in the dramatic topography surrounding its lake-filled valleys.

The centermost valley on Mount Desert Island is connected to the sea. This deep valley, known as Somes Sound, represents the only fjord-like feature on the Atlantic Coast of the United States. The unique features of Somes Sound include its extreme depth (50 m/150 ft.), a shallow sill at its entrance (10 m/30 ft deep), and a hanging valley waterfall on the west shore (Man o' War Brook).

Lowell and Borns (1988) mapped the surficial geology of Mount Desert Island. Deposits of till and glacio-marine sediments (the Presumpscot Formation) are discontinuous and limited to low-lying areas of the park of less than 25 m (90 ft) in present elevation (Kahl et al., 1985). These deposits tend to be found in locations that were protected from the energy of the open ocean during the period of rising sea level immediately following deglaciation. The presence of Presumpscot formation in Maine watersheds has recently been shown to increase the trophic status of lakes in Maine (Nieratko et al., 1992). In the park, this formation generally occurs in valley stream watersheds, and less commonly in lake watersheds, due to the higher elevations in lake watersheds.

Extensive areas of bare rock or areas with only a thin veneer of surficial glacial/soil material occur on all of Acadia's upland areas and ridges, and along much of the shoreline. The absence of soil over significant portions of the park landscape is an important characteristic of the hydrologic environment, leading to poor infiltration of rainwater, rapid surface runoff, stream flows that rise and fall quickly in response to precipitation, and low alkalinity and nutrient concentrations in surface waters.

Soil Characteristics.

Soils are an important component of the hydrologic environment. Soils store and govern the transport of water, as well as contribute cations, anions, dissolved organic carbon and other constituents to ground and surface waters through decomposition and weathering processes. The USDA Natural Resource Conservation Service (formerly Soil Conservation Service) has recently mapped Acadia soils. The predominant soil classification is a shallow-to-bedrock, stony Schoodic-rock outcrop-Lyman complex, derived from granite and schist tills. This classification includes extensive areas of exposed bedrock, along with areas where soils exist as thin deposits of gravelly sandy loam less than 15 cm (6 in) deep (Schoodic soils) and areas where soils form a black and reddish sandy loam less than 50 cm (20 in) deep (Lyman soils). This soil complex is described as excessively well drained, with slopes that range from 0 to 100 percent for bare rock, and from 0 to 80 percent in areas with soil. On steep slopes these soils are usually droughty. Lyman and Schoodic soils are Spodosols -- acidic forest soils characterized by an accumulation of Fe, Al, and organic matter in the B horizon.

Soils in valleys at Acadia are typically part of the Hermon-Monadnock-Dixfield complex. These are mainly sandy loams derived from granite and schist tills. They range from excessively to moderately well drained with regolith depths less than 5 m (15 ft) and slopes up to 60 percent. Below elevations of approximately 25 m (90 ft), these soils are commonly underlain by, or developed from, glaciomarine silts and clays of the Presumpscot formation.

Organic soils, known as Lithic Borofolists, are common on bedrock where mineral soil is absent. Depending on orientation and slope, Lithic Borofolists may range from poorly to excessively drained. The drainage characteristics influence vegetation types and surface water chemistry. Organic soils are also associated with wetlands. A range of poorly to well (fibric to sapric) decomposed peats are present, with fibric peats found in raised bogs such as Big Heath near Southwest Harbor. Sapric peats are associated with forest, shrub fen and emergent shrub/fen communities (Calhoun et al., 1994).

The Lyman soils are traditionally considered low productivity soils even for forestry land use (Rourke et al., 1978). These soils are stony, excessively drained, and have a low cation exchange capacity (CEC). In these droughty soils, the contact time for equilibrium between soils and solutions is often insufficient for significant base cation release from weathering reactions. As a result, the nutrient ions and CEC are derived mainly from organic matter overlying mineral soils or bedrock and the organic carbon reserves in the mineral soil horizons.

Properties of park soils that are of greatest significance to the hydrologic environment include: 1) their shallow depth which reduces infiltration and increases surface runoff in watersheds following storm events, 2) their poorly buffered, acidic nature, and 3) the steep slopes which further hasten runoff. Precipitation percolating through the shallow mineral or organic soils in the steeper upland watersheds becomes enriched in weak organic acids which may contribute acidity (hydrogen ions) to solutions. Aluminum, potentially toxic to terrestrial and aquatic organisms, may be mobilized from soils, rocks, and stream sediments in this acidic environment. When thicker mineral soils are present, acidic soil solutions in the upper soil profile are neutralized as they react with the constituents of the B and C horizons of mineral soils. As a result of these processes, the ephemeral upland streams have the lowest pH and exhibit greater episodic acidifications than perennial streams in the valleys (Kahl et al., 1985). Lake alkalinity tends to be intermediate between the upland brooks and valley streams.

Lake Sediment Chemistry. Lake sediment records from The Bowl, Sargent Mountain Pond and Long Pond on Isle au Haut indicate that atmospheric deposition of elevated concentrations of trace metals has been occurring for over 150 years at Acadia (Kahl et al., 1985; Norton and Kahl, 1987, 1991). Similar records from peat cores taken at the park corroborate this inference (Norton et al., 1997; Norton and Kahl, 1986; 1987). The presence of polluted precipitation 150 years ago suggests that anthropogenic contributions of mineral acids (from fossil fuel combustion) were probably present in precipitation in the mid-1800s as well. The chronology of these cores was determined using 210Pb (Binford et al., 1993), as part of a regional effort to determine the historical impacts and trends in acidic deposition.


Estuarine waters constitute the transition zone between the freshwater and the marine environment. Acadia estuarine wetlands consist of intertidal mud flats, coarse gravel shores, salt marshes, and aquatic beds in coves or embayments, sheltered from high energy waves of the open ocean. The high tidal range at Acadia (over 3m./10 ft.) has helped to create an extensive system of mud flats that are of great ecological and economic importance to the region (Calhoun et al., 1994).

Although estuarine waters are outside park boundaries, the National Park Service has sponsored water quality research on Somes Sound (Doering and Roman, 1994) and Bass Harbor Marsh (Doering et al., 1994). A large part of the watershed for both estuaries is contained within the park. Both estuaries receive freshwater from private and park lands.

Somes Sound is classified as a fjord-type estuary because of its long (8 km/5 mi) and narrow (1 km/.6 mi) configuration, deep basins (40-50 m (130-160 ft) deep), and relatively shallow sill (10-12 m/33-40 ft) deep) near Northeast Harbor (Doering and Roman, 1994). Prior research conducted on Somes Sound includes water chemistry studies by Ketchum and Cass (1986) and hydrology studies by Folger et al. (1972).

Bass Harbor Marsh is a 22 hectare (54 acre) marsh-dominated estuarine system with a main creek that meanders for 3 km (2 mi). A study of water quality and habitat of the marsh ecosystem were initiated by the National Park Service in response to concerns about decreasing brook trout populations, increasing macroalgae biomass, and the knowledge of nitrogen leaching from a landfill in the headwaters outside of the park boundary. The researchers' findings and management alternatives for Somes Sound and Bass Harbor Marsh are summarized in section 3.3.

Source National Park Service, Water Resources Division


Binford, M.W., K. Hill, and F. Ignatowski. 1989. Long-term human activities and ecosystem responses: progress report to the Aga Khan Foundation. Harvard School of Design , Cambridge , MA . Chapman , C.A. 1970. The Geology of Acadia National Park . The Chatam Press, Old Greenwich , CT. 128 pp.

Gilman, R.A. , C.A. Chapman, T.V. Lowell, and H.W. Borns, Jr. 1988. The geology of Mount Desert Island - a visitor's guide to the geology of Acadia National Park . Maine Geological Survey, Department of Conservation, Augusta , ME. 50 pp. + maps.

Kahl, J.S., J.L. Andersen, and S.A. Norton. 1985a. An evaluation of the Technical Report for Management: Water resource baseline data and assessment of impacts from acidic precipitation, Acadia National Park , ME.

National Park Service Document NARO Technical Report 16. 14 pp. Norton , S.A. and J.S. Kahl. 1987. A comparison of lake sediments and ombrotrophic peat deposits as long-term monitors of atmospheric pollution. IN New Approaches to Monitoring ° Aquatic Ecosystems, ASTM STP 940, T. Boyle (ed.), American Society for Testing and Materials, Philadelphia, PA. pp. 40-57.


from the Park's enabling legislation:

Proclamation No. 1339 of July 8, 1916 (39 Stat. 1785) p. 16: "The topographic configuration, the geology, ... are of great scientific interest"

Written in the Rocks
Bedrock Foundations

Acadia's landscape had its beginnings long before sunbeams first caressed the gentle slopes of Cadillac Mountain. About 500 million years ago nameless rivers transported sand, silt, and mud onto the floor of an ancient sea. These sediments built at a rate of about one inch every hundred years until they accreted to depths of thousands of feet. Pressure and heat transformed these sediments into the earliest bedrock. Next, titanic forces lifted and warped the bedrock of the sea into a mountain range, a range perhaps as mighty as the Rockies. But inexorably, the forces of air, water, and gravity ground these mountains down until little was left. Today, only schists and gneisses, rocks of the Ellsworth formation, remain as testimony to those mountains of long ago.

The pattern of deposition repeated itself. Rivers laden with rock and mud poured their cargoes into the sea, which amassed as gravel and silt beds on the ocean floor. Pressure transformed the gravel into conglomerate and the silt into siltstone. Known as the Bar Harbor formation, these deposits roofed over the Ellsworth formation.

Then, about 400 million years ago, volcanoes belched out their contents of ash which came to rest on a sea bottom. During the time when seaweeds dominated the plant world, pressure and heat transformed these sediments into rocks known as the Cranberry Island formation.

Geologists sometimes refer to the Ellsworth, Bar Harbor, and Cranberry Island formations as "weak" rock. This rock, however, is not weak, except when compared to granite (the rock most commonly found in Acadia), which is much more resistant to erosion.

After a period of quiet, molten rock (magma) invaded and reshaped the weak rock formations. The first of these intrusions produced diorite. Then followed three enormous bodies of magma that solidified into granite:

  • first a fine-grained,
  • then a coarse-grained, and
  • finally a medium-grained granite.

Each of the intrusions altered the overhead bedrock chemically and physically, but the most dramatic change occurred when the coarse-grained granite formed.

Far below the earth's surface, a huge molasses-like plug of magma at least eight miles in diameter moved upward. And, as it undermined the overlying bedrock, the heavy roof bedrock began to sag, eventually sinking and melting into the magma. The fiery mass incorporated material from the earlier granites, from diorite, and from the weak rocks. Eventually it reached the surface, and gradually cooled to form the coarse-grained granite.

After the various granites developed, other minor intrusions of molten material occurred. The most conspicuous of these consisted of black diabase dikes that spread themselves into open fractures on older rock. The most obvious examples of these formations in the park are on Schoodic Peninsula.

The story of geologic history rushed on, past the appearance of the first insects, even beyond the time when dinosaurs dominated all of life. During this interminable period of time, the agents of erosion, rivers, rain, weathering, removed the overlying rocks from Mount Desert. The various granites, more resistant to erosion, emerged to form a mountainous ridge with V-shaped stream valleys cut into the face of the range. The stage was set for glaciation, an event not far removed from our own time, at least in the reckoning of geologic eras.

Odyssey of Ice

During the last two to three million years, 20 to 30 ice sheets intermittently covered most of New England. Because each succeeding glacier scraped away signs of earlier glaciations, it is the last glacier that has left the most pronounced impact on today's landscape. The last glacier, between 3,000 and 9,000 feet thick, expanded out of Canada and spread across New England. When the leading edge of the glacier reached the highlands of Mount Desert, the ice surged like dough rising over the lip of a pan, into notches in the mountain crests, until six or eight tongues of ice probed the stream-cut valleys. The ice tongues moved like very slow rivers; but, even at the speed of only a few inches or a few feet per day, the strength of the ice far surpassed that of any river. Not even granite could resist the glacier. Just as a river intensifies its erosional power in the narrows, so did the ice multiply its capacity to excavate in the valleys. Eventually, after the ice melted, some of these deepened valleys became water basins. These water-filled hollows became Eagle Lake and Echo Lake. In one instance, the glacier cut a trough that resulted from an ice-sculpting "binge," so deep that it filled with sea water. This formed a fjord now called Somes Sound.

After the ice forged through the valleys, its main body rose high over the mountain range. On the northern slopes the grinding mass of ice streamlined and rounded the profile of the mountain range from base to summit, while on the southern slopes, the glacier plucked fractured rock from the cliff sides, leaving them jagged and precipitous, a dramatic landscape for visitors of the future to view.

The great ice sheet did not travel alone. Embedded within its mass was a passenger load of sand, stone, and grit, tools with which the glacier etched scratches in the granite or polished its rough face smooth. On its broad back, the glacier conveyed boulders as big as trucks, plucking them from mountain ridges, hauling them along, and dumping them one by one at places distant from their origins. These misplaced boulders are called glacial erratics. The most conspicuous of these is the mammoth rock that rests near the summit of South Bubble Mountain.

Then, about 18,000 years ago, the ice mass stopped growing. It had reached the continental shelf, 150 miles or so south of Mount Desert Island, when a warming trend halted its progress. Cliffs of ice broke from its snout and rivers gushed from its massive bulk. The leading edge of the glacier was now its tail, retreating and reaching central Maine about 4,000 years after having conquered the continental shelf.

Meanwhile, the sea level rose as the glacier melted, flooding today's coastline to a depth of about 300 feet. The earth's crust, freed of its burden of ice, started to rebound. The sea, now unable to hold its position on the land, retreated. The resilient land continued to rise relative to the sea until about 10,000 years ago, when it finally stabilized. Since that time, the level of the sea worldwide has risen to its present height, and continues to rise at a rate of about two inches per century. The rising sea and depressed land mass created a "drowned coast". This means that what appears today as arms and fingers of the sea were once river valleys; islands were the tops of mountains; headlands and peninsulas were rocky ridges.

Shore Patterns
The bedrock gave substance and the glaciers gave character, but without the sea, Acadia would be like a gem without a setting. Each headland, bay, and inlet reveals the majestic interface between sea and land. Acadia's rocky headlands bear the brunt of enormous energies unleashed in waves that batter her cliffs and erupt in lofty spray.

Thunder Hole is a familiar example of the awesome power of the sea. When the wind is strong, the rising tide surges into the narrow chasm, compressing the captured air, and resounds with a boom that is felt as well as heard. The surging tides throw stones, some as large as bowling balls, on the chasm floor. These stones are hurled against the bottleneck of rock in a ceaseless effort to tunnel deeper. (Thunder Hole, however, does not always thunder. Often when waves are small and the tide is low, Thunder Hole remains disappointingly still.)

The sea destroys and displaces, but it also builds. What the sea takes from one point on the coast may be added to another. With the irresistible energy of hammer blows, waves dislodge rock particles, smooth them, and deposit them at the head of nearly every cove. In still other places, the dispossessed stones and cobbles become gravel bars and shoals. Bar Harbor was named for just such a bar, which connects it to Bar Island.

Because the coast is young, sandy shores are rare. But at Sand Beach, shore currents have shifted the tons of sand that the sea eroded from the rocks. Mixed into the sand are broken bits of shells and the skeletons of crabs, mussels, sea urchins, and other marine life.

The story that began with sediments piled on the floor of a primordial sea closes for the moment with those washed ashore at Sand Beach. But in reality there is no beginning and no ending. Rock becomes sand, and sand becomes rock. The granite of Cadillac Mountain, the cobbles at Hunters Cove, even a pinch of grit at Sand Beach bears evidence of this endless cycle. For indelibly written on the landscape, in bold stokes or fine scratches, is a script that tells the astonishing story of mountain ranges that rose and fell, of ice that sealed in a continent, and of coastlines that emerged and vanished.

Glaciers and the Landscape
Effects of continental glaciation abound here. Mount Desert Island's major valleys all run north and south and each holds one or two lakes the glaciers scooped out. Erratic boulders sit where the mammoth ice sheet (two miles thick in places) stranded them. Many rocks are polished or scratched by the glaciers. Somes Sound, the only fjord on the east coast of the United States, is a glacial river valley drowned by the sea.

block diagram of landscape change by ice sheet

from left to right:

  • Before glaciation, the Acadian highland was an east-west running granite ridge
  • Huge ice tongues invaded the valleys on the north slopes, cut through the passes, and engulfed the mountains.
  • The ice sheet swept seaward, finally extending 300 miles out to the continental shelf.
  • As the ice melted, the ocean level rose and flooded the foothills, creating today's indented shoreline.
  • Acadia's shape was changed, U-shaped valleys now run north and south. Its mountaintops are rounded.

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A geology photo album has not been prepared for this park.


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Lillie, Robert J., 2005.
W.W. Norton and Company.
ISBN 0-393-92407-6



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Park Highlights

NPS Geology and Soils Partners

NRCS logoAssociation of American State Geologists
NRCS logoGeological Society of America
NRCS logoNatural Resource Conservation Service - Soils
USGS logo U.S. Geological Survey

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