• American Camp parade ground looking west

    San Juan Island

    National Historical Park Washington

There are park alerts in effect.
show Alerts »
  • Park on Spring Schedule

    The American Camp visitor center is open 8:30 a.m. to 4:30 p.m.Wednesday-Sunday. The English Camp contact station is closed through May 31. Grounds at both units are open from dawn to 11 p.m. More »

Geologic Formations

Mt. Finlayson's glacial terraces.
American Camp's Mt. Finlayson's glacial terraces are illustrative of more than 18,000 years of geologic history. Each step indicates the ancient seashore as, via glacial (or isostatic) rebound, the island rose as the mile-high glacier melted.
Mike Vouri Photo
 
The sequence of rocks in the San Juan Islands is similar to that found to the east in the North Cascades and to the west on the east side of Vancouver Island. San Juan Island is covered with three major deposits and bedrocks. Both American Camp and parts of English Camp are covered with Quaternary alluvium and glacial deposits. Parts of English Camp are covered with Middle to Upper Paleozoic sedimentary and volcanic rocks. Other areas of San Juan Island are covered with Jurassic-Cretaceous sedimentary and volcanic rocks. The San Juan Islands are part of a small plate that rides atop the Pacific plate. The Pacific plate is denser and heavier and slides under the lighter North American plate. Washington is comprised of many small, light plates that were pushed up against the North American plate as the Pacific plate slid under the continental plate. The force of impact squished and metamorphosed rocks of these smaller plates. Thus many of the original sedimentary and volcanic rocks of the San Juans were transformed into metamorphic rocks.

Did You Know?

island marble butterfly

The island marble butterfly was once thought extinct until it was spotted on the American Camp prairie in 1998. The park, in cooperation with the U.S. Fish and Wildlife Service actively monitors the butterfly. More...