The Geology of Pictured Rocks National Lakeshore: Stained Sandstone Cliffs

Pictured Rocks National Lakeshore lies along the southeastern shore of Lake Superior in Alger County, Michigan. Established on October 5, 1972, it covers about 29,637 hectares (73,236 acres) of multicolored sandstone cliffs, long beaches, sand dunes, waterfalls, inland lakes, wetlands, hardwood and conifer forests, and a diverse community of living things.

The "pictured rocks"

The park's namesake cliffs stand 15–61 m (50–200 ft) tall along more than 24 km (15 mi) of shoreline, and the waves have carved them into sea caves, arches, blowholes, turrets and stone spires. The name comes from the streaks that stain the cliff faces where groundwater seeps through the rock and runs down it, carrying minerals:

Miners Castle, 2015. National Park Service.

The rocks beneath

The bedrock is mostly sedimentary and comes from two widely separated stretches of time: the Mesoproterozoic, Cambrian and Ordovician, and the late Quaternary.

• Jacobsville Sandstone (Mesoproterozoic), the oldest rock in the park, is found in its northern part.
• Munising Formation (late Cambrian) makes up much of the park and forms the "Pictured Rocks" along the shore.
• Au Train Formation (Early Ordovician) lies inland, in the southern part of the park.
• Glacial deposits. Through the Pleistocene, glaciers advanced and retreated across the Great Lakes region, leaving till and outwash. Meltwater cut channels into the Cambrian bedrock, including those now followed by Chapel Creek and the Mosquito River.

The lakeshore belongs to the Central Lowland physiographic province and shares its geologic history and several of its formations with a much larger region.

Dunes form a steep stretch of lakeshore. National Park Service.

The NPS Geologic Resources Inventory's map and report for the park, and an NPS soil survey, are available through the NPS DataStore.

Sources

• National Park Service: "NPS Geodiversity Atlas—Pictured Rocks National Lakeshore, Michigan," citing Anderson (2017).