The Geology of Mount Desert Island
A Visitor's Guide to the Geology of Acadia National Park
Bedrock Geology of Mount Desert Island
The Rocks of Mount Desert Island
All three major kinds of bedrock: igneous, sedimentary, and metamorphic, are found on Mount Desert Island.
Igneous rocks are formed by the cooling and crystallization of molten rock, called magma, that forms deep within the earth's interior and then rises toward the surface. Some of this magma cools and solidifies before it reaches the surface, forming an intrusive igneous rock such as the granite on Cadillac Mountain. Other pockets of magma may reach the surface, erupting as extrusive igneous rock, perhaps forming a volcano. Much of the rock exposed on the Cranberry Isles is extrusive.
Layers of sediment form when small particles of rocks and minerals carried by a river are deposited in the ocean, a lake, or along the river bed itself. This material, if eventually buried by additional sediment, becomes compacted into sedimentary rocks. Sedimentary rocks are composed of layers, or beds as geologists refer to them, of different mineral composition or grain size. The Bar Harbor Formation, found along the shore path in the town of Bar Harbor, provides an easily accessible example of bedding in sedimentary rocks.
Some rocks eventually become buried several miles below the earth's surface. High temperatures and pressures at these depths cause the minerals to recrystallize, forming metamorphic rocks. The changes that take place are collectively known as the process of metamorphism, and the product of these changes is a metamorphic rock such as the Ellsworth Schist, found along the north and west sides of Mount Desert Island.
The following sections describe the different kinds of bedrock found on Mount Desert Island and summarize the current interpretation of the island's geologic history. This text should be used in conjunction with the accompanying bedrock geologic map (pdf format) which shows the rock types and features that are found around the island. Each type of rock is identified by color on the map and described briefly in the map explanation. To use the map and text to best advantage, we suggest you first find your location on either the bedrock or surficial geologic map and determine the rock or sediment type by referring to the map explanation, then refer to this text for a more detailed description of the rock and an account of its history. Or, you may choose to read the text and examine the maps in some detail before visiting some of the sites of geologic interest marked on the maps.
Not all of the rocks on the island formed at the same time in the geologic past. They record the geologic history or sequence of events that took place in this part of the earth over a span of many millions of years. Before we turn to the rocks themselves, let's consider the vast span of time they represent. For most of us, ten years is a long time, a hundred years is longer than we expect to be alive, and a thousand years is ancient history. But all these lengths of time are but fleeting instants compared to the span of time over which the rocks of Mount Desert Island evolved. Figure 1 is a chart of geologic time detailing the last 600 million years, thus covering the ages of the rocks on the island. The earth is believed to be about 4.5 billion years old, so there is still a lot of earlier earth history (Precambrian Era) that is not represented by rocks in this area.
As you read the following descriptions, remember that each of the different rock types described below appears as a different color on the geologic map and each is identified and briefly described in the legend. The descriptions begin with a discussion of the oldest rocks on the island and proceed to progressively younger units.
Last updated on January 11, 2008