The Geology of San Juan Island National Historical Park, Washington

San Juan Island National Historical Park lies on San Juan Island, the second largest island in the San Juan Archipelago of San Juan County, Washington. The National Park Service's Geodiversity Atlas, drawn from its Geologic Resources Inventory, summarizes the park's rocks, landforms and soils for education, geoconservation and management.

The park

Authorized on September 9, 1966, the park protects approximately 868 hectares (2,146 acres), including more than 10 km (6 mi) of shoreline, trails, prairies and military camps. It preserves the American and English Camps and commemorates the peaceful settlement of the San Juan Boundary Dispute between Great Britain and the United States, which ran from 1853 to 1872 and included the Pig War crisis of 1859.

Both camps were set up in 1859, after the killing of a pig set off a border dispute that nearly brought the two countries to war, and both stood through the 12-year joint occupation. In 1872 Kaiser Wilhelm I of Germany awarded the San Juan Islands to the United States. The park shows how individuals and governments can avert war and keep the peace.

The park's Geologic Resources Inventory geologic map. NPS.

A landscape built by collision and ice

Three kinds of process shaped the island: plate collision, ice-age glaciation and the modern coastline's work.

• Collision. Paleozoic and Mesozoic rocks — the Garrison Schist, the Decatur Terrane and the Deadman Bay Terrane — record thrust faulting and metamorphism from the collision of the North American and Farallon Plates in the Cretaceous, approximately 84–100 million years ago. The convergence deformed masses of bedrock, pushed them against one another and carried them about 2,500 km (1,600 mi) north along the western coast of North America.
• Glaciers. Pleistocene ice covered the island approximately 17,000 years ago, carving and rounding the harder bedrock. Melting and retreating, it left poorly sorted, loose deposits of gravel, sand and silt.
• Rebound. Freed of the weight of ice and snow, the land slowly rose — isostasy — lifting old beaches into exceptionally well-developed terraces.
• The coast. Since the ice ages ended, onshore winds, tides, rip currents and longshore currents have kept reshaping the shoreline.

The park belongs to the Pacific Border physiographic province, sharing its geologic history and some characteristic formations with a region well beyond its boundaries.

The Redoubt and the prairie beyond. NPS.

Maps and inventories

The Geologic Resources Inventory produces digital geologic maps and reports for more than 270 natural resource parks: scoping summaries recording the mapping plan agreed by park staff and local geologists, GIS map data, reports on each park's setting, features, management issues and history, and posters. San Juan Island's products are in the NPS DataStore, along with a completed NPS Soil Resources Inventory for the park.

Sources

Rewritten from the National Park Service article "NPS Geodiversity Atlas—San Juan Island National Historical Park, Washington" (public domain), in hubnx's own words. The source spells isostasy "isostacy"; the report cover it shows is not reproduced.