The Geology of Mount Desert Island
A Visitor's Guide to the Geology of Acadia National Park
The older rocks on Mount Desert Island are stratified and include both sedimentary and volcanic types, the oldest of which have been substantially metamorphosed, while the younger ones are relatively unaltered.
The very first rock you see along the shore as you cross the bridge onto the northern end of Mount Desert Island is called the Ellsworth Schist (labeled
COe on the bedrock geologic map - pdf format). It is the oldest rock exposed on the island. Large outcrops are found at the picnic area opposite the Acadia National Park information center on Thompson Island; on the northwest side of Mount Desert Island; and on the west side of Bartlett Island. The name schist refers to a type of metamorphic rock that is layered in appearance and that breaks along this layering. You may be more familiar with slate, another metamorphic rock that has these same properties. A schist, however, is more irregular in the way it splits than a slate. In addition, the mineral grains are larger in a schist, the grains of a slate being microscopic in size.
The Ellsworth Schist is commonly dark green or gray, and often has a distinctive streaked appearance caused by thin, lighter colored layers of quartz and feldspar mixed with darker layers consisting mostly of the mineral chlorite. The small barbed symbols on the bedrock geologic map show the orientation of this layering or foliation. The small wedge on the symbol points down the slope of the layers, in the direction water would drain. In many places you will find this layering folded into intricate patterns (Figure 2).
The complicated appearance of the Ellsworth Schist records its complex geologic history. The rocks originated over 500 million years ago as deposits of mud on a sea floor. Volcanic activity in the area at this time is indicated by the composition of certain layers within the schist that resemble modern day volcanic rocks. These sediments were subsequently buried several miles below the earth's surface where higher temperatures and pressures brought about the changes of metamorphism including recrystallization and folding. We assume that the sedimentary layers were at one time more or less horizontal, so the complicated folds shown in Figure 2 indicate that the rock was deformed by forces acting within the earth. As a result, when erosion finally exposed the rock at the earth's surface millions of years later, it bore little resemblance to the original ocean-bottom sediments. But the rock contains clues that geologists learn to recognize and interpret in order to unravel the events that make up its history. Part of this history involves the startling idea that the Ellsworth Schist was not originally part of North America at all, but rather part of a small "continent" called Avalonia that became attached to North America during a time of continental collision between 350 and 400 million years ago. This is part of the theory of plate tectonics, which we will discuss in more detail under the section on geologic history.
Bar Harbor Formation
The Bar Harbor Formation (DSbh on the geologic map) is believed to be the second oldest group of rocks on the island. Notice in Figure 1 that it is more than a hundred million years younger than the Ellsworth Schist. This gap in the geologic record, called an unconformity, can be explained in two ways. Either no sediments were deposited during the millions of years between the deposition of the sediments that make up the Ellsworth Schist and the Bar Harbor Formation, or, more likely, rocks younger than the schist but older than the Bar Harbor Formation were deposited and then eroded during periods of uplift between the Cambrian and the Late Silurian Periods. The unconformity between the Ellsworth Schist and the younger rocks is currently interpreted as evidence of uplift and deformation that occurred during this period.
Most of the Bar Harbor Formation consists of siltstone and sandstone beds measuring a few inches thick. Where exposed, the beds generally weather to a tan, gray, or lavender color but are typically dark gray on a freshly broken surface. These sediments were also deposited in an ocean environment, but possibly at shallower depths than the Ellsworth Schist sediments. Some volcanic material is also present. The Bar Harbor Formation takes its name from the excellent exposures along the shoreline at Bar Harbor where you can see the bedding inclined gently toward the ocean (Figure 3). The lack of complex folding and the gentle tilt of the beds indicate that these rocks were not subjected to the strong deformational forces that altered the Ellsworth Schist.
Occasionally, the coarser-grained beds contain quartz pebbles up to ¼ inch in diameter. In some cases individual beds display graded bedding, which shows a gradational change from coarse material at the bottom of the layer to fine silt at the top (Figure 4). This feature develops when a mixture of different-sized particles settles in water. The largest and heaviest particles settle out first, forming the bottom of the bed. These are gradually covered with increasingly finer and lighter particles. The cycle is repeated when another mass of sediments enters the water. Graded beds are one of the clues a geologist looks for to determine a rock's history. For instance, if the grains are observed to get finer downward rather than upwards as in Figure 4, a geologist infers that the rock layers have been turned upside down during folding. Graded bedding provides evidence that the rock strata in the Bar Harbor Formation are right-side-up with the same general orientation now as when they were deposited over 420 million years ago.
The rocks found along the shore at The Ovens and in the road-cuts along Route 3 at Ireson Hill on the northern side of the island also belong to the Bar Harbor Formation, but here the rock is flint-like and bedding is difficult or impossible to see. Some of these rocks are believed to be accumulations of ash that settled out of the atmosphere after a volcanic eruption.
Cranberry Island Series
The rocks that form the Cranberry Isles and the southwestern part of Mount Desert Island are largely the result of volcanic eruptions that took place along what is now southeastern coastal Maine about 380-400 million years ago. Three types of rock belonging to the Cranberry Island Series are shown on the geologic map: DScif, felsites and volcanic flows; DScit, volcanic tuffs; and DScis, interbedded volcanic and sedimentary rocks.
Rocks of the Cranberry Island Series are generally light gray or blue-gray and are most easily recognized by layers of volcanic tuff, a type of rock that formed as small pieces of rock debris settled after an explosive volcanic eruption. Good examples can be seen in front of the Seawall picnic ground parking area (Figure 5). By looking closely, you will see that the rock is composed of a variety of angular fragments ranging in size from a fraction of an inch to a few inches in diameter.
Extensive exposures of volcanic tuff also occur along the west shore of Great Cranberry Island. The southern end of Great Cranberry Island has abundant layers of sedimentary rock with only occasional beds of tuff.
Along the ledges on the north side of Little Cranberry Island you will find excellent exposures of rock thought to be old lava flows. Its chaotic appearance is due to the continued flow of the lava as it cooled and solidified. Blocks of solid rock were engulfed in the molten lava which then crystallized around them.
The rocks described above (Ellsworth Schist, Bar Harbor Formation, and Cranberry Island Series) all had their beginnings at the surface of the earth, either as accumulations of sand and mud in the case of the Bar Harbor Formation and Ellsworth Schist or as the products of erupting volcanoes in the case of the Cranberry Island Series. We now turn to the group of intrusive igneous rocks: the gabbro-diorite and granites that formed when magma intruded into the overlying rock, cooled, and solidified deep below the earth's surface.
Last updated on January 11, 2008