The Geology of Valley Forge National Historical Park, Pennsylvania

Valley Forge National Historical Park lies about 29 km (18 mi) northwest of downtown Philadelphia, on the line between Chester and Montgomery counties, Pennsylvania. Authorized as a unit of the National Park System on July 4, 1976, it covers about 1,403 hectares (3,468 acres), including the site where General George Washington and the Continental Army spent the winter of 1777–1778. The park honors the sacrifices and achievements of that six-month encampment, during which the army became a unified, disciplined fighting force, and it protects historic landscapes, earthworks, archeological sites and buildings, among them Washington's Headquarters.

A ceremony commemorating the French Alliance. Image from the National Park Service's page.

The National Park Service's Geodiversity Atlas summarizes the park's geology, drawing on the Geologic Resources Inventory.

The rocks underfoot

Valley Forge is in the eastern part of the Piedmont physiographic province and shares its geologic history with a region well beyond the park. Its rocks span a wide range:

• The oldest are Precambrian gneisses and Paleozoic sedimentary rocks, all metamorphosed and deformed. In outcrops the older Paleozoic units are faulted, folded and metamorphosed to differing degrees.
• In the north, largely flat-lying Triassic sediments of the Stockton Formation, laid down in a Mesozoic rift basin, rest on older Cambrian rocks of the Chickies, Antietam, Harpers and Ledger formations.
• In the center and south, scattered Cretaceous and Cenozoic deposits crop out: the Patapsco, Bryn Mawr, Pensauken and Bridgeton formations.

Maps and reports

The park's digital geologic map. Image from the National Park Service's page.

The Geologic Resources Inventory makes digital geologic maps and reports for more than 270 natural-resource parks. For Valley Forge, its scoping summary, digital geologic maps, report and poster are on the NPS DataStore, as is the park's completed soil resources inventory.

Sources

• National Park Service: "NPS Geodiversity Atlas—Valley Forge National Historical Park, Pennsylvania," citing Thornberry-Ehrlich (2010) and National Park Service (2016). Its image of the inventory report's cover is not reproduced.