The Geology of Mount Desert Island
A Visitor's Guide to the Geology of Acadia National Park
Disappearance of the Glacier
As the earth's climate warmed, Mount Desert Island became ice-free by a process called deglaciation. In some parts of the world, glaciers simply melted under the influence of sun and rain, but in areas such as Mount Desert Island the glacier withdrew in a more complex fashion because it was partly in contact with the sea. The deglaciation history of the island can be divided into three parts which will be described separately: (1) glacial downdraw into the sea, (2) melting of glacial ice on land, and (3) uplift (rebound) of the Earth's crust as the weight of the ice sheet was removed. As we consider these individual aspects of deglaciation, it should be remembered that they are all interrelated.
Glacial Downdraw into the Sea
When the last ice sheet was at its greatest extent, sea level was about 330 feet below its present level. By about 18,000 years ago, however, the climate had warmed enough to begin melting the ice. The melting of the continental ice sheet returned water to the oceans, causing a rise in sea level.
As the sea rose against the edge of the ice sheet, the water eventually became deep enough to float the seaward edge of the ice up off the bedrock. Since it was no longer slowed by friction with the underlying ground, the floating ice began to flow rapidly into the sea and disintegrate into icebergs. This process, called downdraw, may have increased the speed of the glacier in marginal zones by a factor of ten and probably removed large volumes of ice from coastal Maine. However, the downdraw stopped when the ice margin retreated to a position above sea level. On Mount Desert Island this occurred about 13,000 years ago. The ice remaining in areas above sea level was land-based, and deglaciation proceeded in a different manner.
Melting of Glacial Ice on Land
As the climate continued to warm during late-glacial time, the ice surface melted and produced vast amounts of water. Some of this meltwater flowed directly off the surface of the glacier, but a sizable amount found its way to the bottom of the ice. The internal meltwater carved channels within the glacier. We can partly trace these channels because some of them reached the hills beneath the glacier and eroded into the bedrock. The resulting trough-shaped features are called meltwater channels. Glacial streams were able to carve these channels because the strong currents carried abundant sediment, which abraded the bedrock surface. Meltwater channels exist high on the sides of all the mountains on the island. One of the best examples forms the gap between Cadillac Mountain and Dorr Mountain and can be seen from the Park Loop Road where it crosses Otter Cove. These channels were cut prior to the deglaciation of the land.
As the surface of the melting glacier steadily dropped, Cadillac Mountain and other peaks became exposed as islands of bedrock in the ice. However, the ice still flowed around the flanks of the emerging mountains as deglaciation progressed. Lobes of ice extended southward along the major valleys that occur in the central part of Mount Desert Island, such as those now occupied by Long Pond, Somes Sound, and Jordan Pond. The sea was in contact with the ice lobes where they extended from the southern ends of the valleys, but the water depth was not sufficient to float them. Therefore, the ice in these valleys still rested on the ground, and it continued to transport rock debris. The freshly exposed mountain peaks also contributed to the debris load. All of this rock debris was carried along by the flowing ice, as if on a conveyor belt, and was released to form mounds and ridges at the front edge of the ice. These glacial dump piles, called end moraines, can be seen in the southern portions of several valleys (Wm on the surficial geologic map - pdf format).
The best-known end moraine in Acadia National Park forms a natural dam at the south end of Jordan Pond, and the Jordan Pond House was built on its crest. The till that makes up this moraine consists of sediment ranging from clay to boulders (Figure 23). The melting ice also supplied water that transported and sorted some of the dumped sediment into a sand and gravel mixture known as outwash. Boulders moved little, if at all, in the meltwater streams, while the tiny silt and clay particles washed away into the ocean. One of these areas of outwash extends from Bubble Pond southward along Hunter Brook (Wgo on the surficial geologic map). In some places the sand and gravel entering the ocean built up to the water surface, forming flat-topped deposits called deltas (Wgd on the surficial geologic map). The tops of these deltas, such as the one just south of the Jordan Pond moraine, mark the approximate position of sea level when the deltas were formed. At times the glacier advanced a little and overrode the outwash, causing the sand and gravel layers to be pushed and folded. Several folded gravel layers have been exposed in a gravel pit near Southwest Harbor.
A third process that occurred during deglaciation was rebound of the land surface. The crust of the Earth is flexible and floats on the denser, slightly plastic mantle beneath. Any weight added to the crust will push it down. For example, 3,000 feet of ice will cause the underlying bedrock to sink about 1,000 feet. However, when the ice melts, the crust will rise slowly back to its original level. The uplift of the land that takes place during this rebound process may continue for several thousand years following deglaciation.
The solid line in Figure 24 shows the changes in elevation of the earth's crust during a full cycle of glaciation, including the rebound phase described above. The dashed line in this figure shows the worldwide changes in sea level caused by the growth and shrinkage of continental ice sheets. During the cycle of glacial advance and retreat, the relative positions of these curves show whether sea level was higher or lower relative to its present position. For example, during the period of rapid glacial retreat following maximum glaciation, sea level was relatively higher than the present-day land surface, and today's coastal lowlands were drowned by the sea. During the period following rapid crustal rebound, sea level was relatively lower than at present, and today's offshore areas were above sea level. The important conclusion to be drawn from this illustration is that the relative positions of the land surface and sea level - not their absolute elevations - determine whether the coastline was higher or lower than its present position.
It is evident from Figure 24 that even though global sea level was lower as the last ice sheet retreated, the land (which hadn't rebounded yet) was even lower than the ocean. This circumstance allowed the sea to flood much of coastal Maine, including low-lying portions of Mount Desert Island. Elevated shoreline features on the island, such as the delta south of Jordan Pond, indicate that the relative position of sea level was about 230 feet higher during late-glacial time. This raised sea level allowed water to follow the ice margin onto the island, and the sea extended well into lowland areas of central Maine before rebound forced it to withdraw.
In Acadia National Park, the most widespread evidence of submergence by the sea is the discontinuous blanket of marine clay that covers some of the low areas on Mount Desert Island (Wef on the surficial geologic map). This clay consists of very small mineral particles (rock flour) that were derived from glacial abrasion and washed out of the melting ice. Marine clay was once called cove clay because of its concentration in the coves and near-shore valleys of the island. If you look closely, you can see these clay deposits in eroded banks around the coves along Ocean Drive. Marine clay deposits are also exposed on the east side of Otter Cove where it is crossed by the Park Loop Road and at the east end of Sand Beach. Although the clays were originally gray or bluish-gray, most have been weathered to a brown or brownish-gray.
In places the marine clays contain well-preserved shells of clams, mussels, and other mollusks. Some of the fossil shells belong to species that now live only in cold, subarctic waters. These fossils indicate that the late-glacial marine environment of coastal Maine was similar to that of southern Greenland today. The shells can also be used to determine the time of glacial retreat and invasion of the sea. For example, a clay deposit on the shore of Goose Cove on the west side of the island contains shells determined to be approximately 12,250 years old by the carbon-14 dating method.
Other evidence of former marine submergence is the wave-worked boulders, gravels, and sands that mark ancient shorelines well above present sea level. Look at any of the coves along the Park Loop Road and you will see modern beach deposits consisting of boulders, cobbles and perhaps sand lapped against the bedrock cliffs. Similar deposits can be seen at higher elevations in the park, documenting sea levels of the past (Wec on the surficial geologic map). On the east side of Day Mountain, east of the village of Seal Harbor, there are several emerged beaches containing well-rounded boulders (Figure 25). Near this former beach there are also steep, wave cut cliffs such as those found along the present shoreline.
Rebound of the earth's crust soon outpaced the rising sea and forced the shoreline to drop. The ages of fossils from the marine clays mentioned above reveal that Mount Desert Island and the rest of coastal Maine rose above sea level between 12,000 and 11,000 years ago. Rebound continued until the relative position of sea level was about 215 feet lower than it is today. No evidence of this minimum sea level is visible on land, but drowned stream channels and deltas are present offshore in the Gulf of Maine. Continued melting of glaciers worldwide eventually raised sea level to its present elevation.
Last updated on January 11, 2008