Acadia National Park

Geology
Gorp.com

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, twenty to thirty 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 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, 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.


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