The Geology of Mount Desert Island

A Visitor's Guide to the Geology of Acadia National Park

Joints and Faults

As you walk over the ledges of Cadillac Mountain, look carefully at the surfaces of the granite. The many long, straight cracks in the rock that commonly form smooth surfaces are joints, natural fractures in rock along which there has been little or no movement or displacement (Figure 10). Many of these fractures are nearly vertical, and frequently there will be several of them close together and parallel to each other that are referred to as joint sets. Further examination will disclose that there are several joint sets on Cadillac Mountain, the most prominent ones having directions of east-west, 40 degrees east of north, and 25 degrees west of north.

In addition to the vertical joints, there are prominent, nearly horizontal joints which form the broad exposures of smooth, gently sloping rock on the high ridges. These exfoliation joints most likely formed as the weight of overlying rock was removed by erosion, and the rock beneath expanded and cracked parallel to the surface. The presence of both vertical and horizontal joints weakened and loosened the bedrock and allowed the glaciers to remove large blocks of rock.

Joints and faults are due to forces acting within the earth that have at times exceeded the breaking strength of the rock. Under some conditions the forces are great enough to not only fracture the rock but also to cause slippage along the fracture, forming a fault. Faults are not as easily found as joints and are not often seen in outcrops. There are several reasons for this: the limited amount of bedrock outcrop reduces the number of faults exposed at the surface; faults usually contain ruptured and broken rock which weathers easily; and many faults may lie in low areas, covered by soil or glacial material. The presence of a fault is usually inferred from other geologic evidence, such as the abrupt and unpredictable change in rock type or orientation.

Several small faults, recognized by an abrupt interruption of bedding, can be found in the Bar Harbor Formation along the north shore of the island in the vicinity of The Ovens. These are all nearly vertical and are too small to show on the geologic map; none can be traced inland.

A larger fault is inferred at Salsbury Cove where sheared and broken Ellsworth Schist terminates abruptly against the gabbro-diorite. While the actual fault cannot be traced inland, it is possible that the eastern boundary of the Ellsworth Schist south of Salsbury Cove is a continuation of this fault.

Introduction   Bedrock   Stratified Rocks   Igneous Rocks   Structure   Schoodic   Isle au Haut   Bedrock History   Glacial   Erosion   Retreat   Glacial History   Processes   Conclusion   Reading   Glossary   Maps

Last updated on February 13, 2006