The Geology of Mount Desert Island
A Visitor's Guide to the Geology of Acadia National Park
Summary of the Bedrock Geologic History of Mount Desert Island
Our story of the bedrock of Mount Desert Island covers only a small part of the history of the earth: the Cambrian, Ordovician, and parts of the Silurian and Devonian Periods (see Figure 1). What transpired here at other times in the geologic past is not clear because there are no rocks of those ages on the island to preserve the geologic record. As a result, a complete history must be pieced together by examining rocks elsewhere along the Atlantic coast.
The earliest history recorded in the rocks of the park dates back perhaps 550 million years, to the Cambrian Period. At this time the region was part of an ocean floor. Mud, some of which formed from explosive volcanic ash falls, accumulated on the sea floor. Continued deposition of sediments buried the mud well below the earth's surface where it was squeezed and heated by forces acting within the earth. Eventually, uplift and erosion exposed this material at the earth's surface once again, not as mud but as the metamorphic rock we now call the Ellsworth Schist.
Approximately 400 million years ago, during the Silurian and Devonian Periods, sediment once again accumulated on an ocean floor; some as silt and sand, which became the sedimentary rock known as the Bar Harbor Formation, and some as deposits of volcanic ash and lava flows now known as the Cranberry Island Series. Both of these rock units have been folded to some degree, but neither shows evidence of strong metamorphism.
The youngest rocks found on Mount Desert Island are the intrusive igneous rocks. The gabbro-diorite, diabase dikes, and the various granite bodies were emplaced between 360 and 380 million years ago during the Devonian period.
It was mentioned earlier that part of the history of the Ellsworth Schist involved the theory of plate tectonics. In this theory the crust of the earth is conceived as being composed of rigid "plates" which float on the underlying, partially molten mantle. These plates, some of which include the continents, are in constant motion relative to one another. It is the movement of continents as part of these plates that gives rise to the more popular phrase - continental drift.
As a result of detailed geologic studies along the coast of the northeastern United States and the Canadian Maritime Provinces, it appears that the Maine coast as we now know it was not part of North America until the middle of the Paleozoic Era, sometime in the Early Devonian. Maine's coastal region is believed to have been part of another continent, called Avalonia, that collided with North America during the Devonian Period. Evidence for this collision comes from the major differences in rock type, age, fossil assemblages, and structure between portions of coastal Maine and the interior. Very little is actually known about Avalonia. Other remnants of this land mass are found in eastern Massachusetts and in Maritime Canada, where it was first recognized. Avalonia may have been related to the larger continent of Eurasia, or existed as an isolated continental fragment for some time prior to collision with North America.
When two continents collide, they become joined together. In Maine, the Ellsworth Schist, with its long history of metamorphism and folding, was part of the Avalonian continent that was joined to the eastern edge of North America. It is probable that the Bar Harbor Formation and the Cranberry Island volcanic rocks were deposited on top of the Ellsworth Schist, perhaps at about the time the collision was taking place. Thus, the Ellsworth Schist, Bar Harbor Formation, and Cranberry Island volcanic rocks were all added to what previously had been the eastern edge of North America.
This collision of continents accompanied the closing of an ancient ocean basin called the Iapetus Ocean - the forerunner of the Atlantic Ocean. Final closure of the ocean involved the collision of Eurasia with North America in the Late Paleozoic. The present day Atlantic Ocean, on the other hand, started to form a little over 200 million years ago as North America broke away from Eurasia. But this split did not take place precisely at the point where the continents were joined, and a small part of what was once Avalonia was left behind, "welded" to the eastern margin of North America. This sequence of events is diagrammed in Figure 15. Keep in mind that this interpretation is only one of several possibilities, and that there is currently much debate among geologists as to the details of the geologic history. In addition, the history of Mount Desert Island is but a small segment of a larger story that accounts for the development of the entire Appalachian Mountain chain that extends from Newfoundland to Georgia. Geologists use the term orogeny to refer to the process of continental collision and resulting folding and metamorphism of rocks, and, in some cases, the generation of long chains of mountains.
Igneous activity commonly accompanies orogeny, and the region around Mount Desert Island illustrates this well. The process of continental collision must have generated sufficient heat at depths tens of miles below the surface to melt some of the rock there, creating pockets of molten rock. This magma rose toward the surface of the earth, and was injected into the Ellsworth Schist, Bar Harbor Formation, and Cranberry Island Series about 360-380 million years ago. The rock exposed on the bare ledges of the highest mountains on the island, for example, Cadillac Mountain, owes its existence to this period of earth history. Several different types of igneous rocks formed during this time. Listed in order of their intrusion, they are (1) the gray to black, coarse-grained gabbro-diorite; (2) the granite near Southwest Harbor; (3) the granites on and near Cadillac Mountain; and (4) the granite near Somesville. Diabase dikes also formed during this same period of igneous intrusion.
The first section of this guide has outlined the geologic evolution of the bedrock in the Acadia National Park region over a span of about 150 million years. But what of the time from 350 million years ago until the comparatively recent Ice Age? Unfortunately, there are no rocks in the area to record events that occurred during this interval of time. As mentioned above, present theories suggest that the Atlantic Ocean developed during the last 200 million years or so. It continues to widen at the rate of about one inch per year. We also know that extensive erosion took place on land because the granites now exposed on Mount Desert Island could only have cooled and formed several miles below the surface. Any rocks that might have been deposited after the intrusion of the igneous rocks have all been worn away.
The latest major event of earth history that affected the entire park region was glaciation. Thick glacial ice flowed southward across Mount Desert Island a number of times during the last few hundred thousand years, sculpting the landscape that we see today. The remainder of this guide will discuss the history of the island during the Ice Age, as well as the geologic processes that have modified the region since the retreat of the last glacier.
Last updated on January 11, 2008