San Juan Island National Historical Park - Geologic Formations (U.S. National Park Service)

Mike Vouri Photo

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.

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.