Ancient Fault Rocks at Fort Foster Park, Kittery

Introduction

Have you ever been in a fault zone? Some parts of the world are riddled with active faults, places where the earth's bedrock breaks and moves repeatedly. People who live in such geologically active areas, such as Alaska, the Philippines, China, and California, are familiar with the main surface effect of sudden fault motion - earthquakes. But what happens to the rocks below the surface that are caught in a fault zone where earthquakes are generated?

One way to find out is to drill a hole two miles deep into an active fault, as in the San Andreas Fault drilling project sponsored by the National Science Foundation. Another way is to find a place where an ancient fault zone has been eroded, so that rocks which formed at depth can now be seen at the surface. One such place is in southern Maine, at Fort Foster Park. The rocks here were in a major fault zone at a significant depth in the earth when they formed, probably between 270 and 300 million years ago (Swanson, 2007). In the expanse of geologic time since then, the overlying rock has been eroded, allowing us to examine rocks like those that would be found today several miles beneath the San Andreas fault.

Location

Figure 1
Air photo

Fort Foster is at the southern tip of Maine, past the village of Kittery Point. Technically, it is on Gerrish Island, although this "island" is barely separated from the mainland by a narrow creek and wetland, and can be reached easily by car. The park, a former military installation, is owned and operated by the town of Kittery. Visitors are charged admission during the summer season. The rocks of interest are along the southeast shore, below the picnic pavilion (see Figure 1 - air photo).

Important! This is a unique natural exposure that has been used for detailed geologic study and for teaching students. While photographs are encouraged, there is no reason or excuse for breaking or damaging these rocks. Please take care of this special place.

Features of faulted rocks found at Fort Foster

Click on these photos to learn about some of the fault-related features that can be seen at Fort Foster Park.

Figure 2
Foliation

Figure 3
Crumpled layers

Figure 4
Offset layers

Figure 5
Pseudotachylyte

Geologic Slide Show of Fort Foster Fault Rocks

Now that you have learned what to look for, take a photo tour of the Fort Foster site by viewing our slide show. Please be patient as Java applets and associated images are downloaded to your computer.

For More Information, See These References

Hussey, Arthur M., 1980, The Rye Formation of Gerrish Island, Kittery, Maine (pdf format): The Maine Geologist, v. 7, no. 2, p. 2-3.

Swanson, Mark T., 1988, Pseudotachylite-bearing strike-slip duplex structures in the Fort Foster Brittle Zone, southern Maine: Journal of Structural Geology, v. 10, p. 813-828.

Swanson, Mark T., 2006, Pseudotachylyte-bearing strike-slip faults in mylonitic host rocks, Fort Foster Brittle Zone, Kittery, Maine. In Abercrombie, R., McGarr, A., Kanamori, H., and DiToro, G., editors, Earthquakes: Radiated energy and the physics of faulting: American Geophysical Union Monograph Series, vol. 170, p. 167-180.

Swanson, Mark T., 2007, Structure of Late Paleozoic brittle dextral strike-slip faults in coastal Maine exposures. In Peter Thompson (editor), Guidebook to field trips in New Hampshire, adjacent Maine & Massachusetts: Northeastern Section, Geological Society of America, Durham, N.H., Trip F-2, p. 3-18.