Geology and History of Mount Desert Island

Community Ecology of Coastal Maine homepage

Geology and History of Mount Desert Island

Birth of coastal Maine mountains
Glacial weathering
The year the park burned

A given biological community is the result of biotic (living) interactions between species adapted to survive in a given habitat, and the abiotic (non-living) parameters that define that habitat. At each site visited, we describe the variety of species observed, and some of the immediate abiotic factors (slope grade, temperature, soil depth, humidity, wind speed, etc.). However, it is also worthwhile to briefly describe the forces that over geological and recent history, have literally shaped Mount Desert Island as we know it today. These factors have had, and continue to have, a strong influence on the make-up and disposition of the biological communities of Mount Desert Island.

Birth of coastal Maine mountains
Approximately 400 to 360 million years ago (MYA), what is now the coast of the U.S. collided with what is now Europe, forming part of the super continent Pangea. This collision pushed up a series of mountains, in much the same way that a collision between two cars might ripple and bend up the hood of a car. Later, approximately 200 MYA, as Pangea broke apart, the Atlantic ocean formed, and part of the newly formed mountain range ended up along the eastern edge of this new ocean. This stretch of mountains became the coast of Maine, including Mount Desert Island.

Glacial weathering
The more recent (200 million years) geological history of coastal Maine is dominated by a series of ice ages, and the weathering of the glaciers spawned by these ice ages. Coastal Maine shows only the evidence of the most recent ice age, 40,000 to 12,000 years ago, because it destroyed any evidence of earlier glacial periods. Signs of this most recent ice age and of glacial weathering can be seen everywhere on Mount Desert Island, indeed the very shape of the island, its mountains, valleys, and bodies of water are all due to glacial weathering.

U-shaped valley - The parallel u-shaped valleys of Mount Desert Island were carved by glaciers as they filled the existing stream-formed v-shaped valleys. The great weight of the glaciers scoured these valleys into deep u-shapes as the glacier moved through (see Figure 1).

Fjord or lake - Once the glaciers had receded, the fate of the valleys left behind depended in part on the height of the moraine, a great pile of material pushed ahead of the moving glacier and left in place when it receded. If the moraine was above sea level, the valley might remain as dry land, or might fill up with fresh water, forming a lake, pond, or tarn (see Figures 2 and 3). There are several such valleys on Mount Desert Island, including the ones containing Eagle Lake, Jordan Pond (link to jordanpond6.jpg), the Long Pond. If the moraine was shorter, and fell below sea level, then the sea would flood it, and a fjord would form (see Figures 2 and 3). Somes Sound is the only fjord on the east coast of the United States.

Smooth or jagged - As a glacier grew and moved forward, it would scour smooth the ground beneath it. If a glacier grew high enough to cross over the top of a mountain range (during the most recent ice age, the glaciers towered up to 2 miles above the tops of the mountains on Mount Desert Island) it would scour smooth the up-hill side of the mountain, a process called abrasion (see Figure 4). On the down-hill side of the mountain, however, the glacier would often tear away pieces of the hillside as it flowed away from the mountain, a process called plucking (see Figure 4). Signs of abrasion and plucking can be seen all over the island. In fact, the island got its name, L'Isle des Monte Déserts, French for "island of the deserted mountains" because the tops of the mountains were largely without trees. This lack of trees was the result of glacial abrasion scouring the mountaintops down to bare stone, leaving no soil for plants to grow in.

Erratics - At times, rocks were picked up by glaciers and moved great distances from where they were formed to be dropped as the glacier melted. These misplaced rocks are called erratics. One rather famous erratic is Balance Rock on the South Bubble. This huge, weathered rock sits on the edge of a cliff nearly atop the South Bubble and is clearly not formed from the local pink "cadillac" granite.

The year the park burned
In more recent history, during the fall of 1947, a large portion of Acadia National Park burned as a forest fire raged across much of Mount Desert Island. Though this was an isolated even it merits mention because of the great impact it had on the make-up of the forest communities on the island. Prior to this fire, the island was largely covered by boreal forests dominated by spruce trees. In this region, boreal or spruces forests are considered climax forests, the end result of hundreds, perhaps thousands of years of gradual succession (change in species composition). Spruce forests have a very low species diversity, in part because they let in very little light and as consequently, few other plants can grow here. Though the fire cause a lot of damage to the park, it also acted as an ecological disturbance, opening large stretches of terrestrial habitat to a wide variety of species and increasing species diversity on the island. Many places on the island are now a patchwork of many species of trees, shrubs and undergrowth. Without the change wrought be this fire, most of the hardwood forests we visited would not exist).

Bibliography
Caldwell D.W. 1998. Roadside Geology of Maine. Mountain Press Publishing Company, Missoula, Montana.