Apostle Islands' Geology: Billion-Year-Old Sandstone, Carved by Ice and Lake Superior

The sea caves of Apostle Islands National Lakeshore, which freeze into ice-fringed grottoes in winter, are cut into sandstone laid down more than a billion years ago. Between that rock and the loose sediment on top of it lies a gap in the record of nearly 1.1 billion years.

An opening in an ice-fringed sea cave, with ice floes on Lake Superior, winter 2008. NPS.

The park

The lakeshore takes in 21 of the 22 islands in southwestern Lake Superior, in Ashland and Bayfield Counties, Wisconsin, plus 19 km (12 mi) of mainland shore on the Bayfield Peninsula — about 28,075 hectares (69,377 acres), established on September 26, 1970. It holds sea cliffs and caves, terraces, wave-cut benches, arches, beaches and old-growth forest; inland, glaciers left grooves and striations in the bedrock and thick blankets of sediment.

Nobody is sure where the name comes from. Early French explorers may have named the islands after the twelve apostles — or it may recall a band of thieves who used the straits, caves and coves to raid fur traders' cargoes.

A rift that failed

The bedrock is the Bayfield Group of the Mesoproterozoic — the Orienta, Devils Island and Chequamegon sandstones — deposited on sand flats, in braided streams and in shallow lakes inside the Midcontinent Rift System, a rift that began to split the ancient core of North America and then failed. Volcanic rocks from the rift lie buried beneath the sandstone.

Carved by ice

More than a billion years later, Pleistocene glaciers advanced and retreated across the Lake Superior region, eroding the bedrock around resistant knobs — the future islands. They left glacial deposits, including the Miller Creek and Copper Falls formations, and shoreline sediments settled on the old sandstone after the ice.

Geologic map of the lakeshore, from the Geologic Resources Inventory. NPS.

The park is part of the Superior Upland physiographic province. Its fossils and caves are protected by federal law, and old mine sites, where they occur, can be hazardous — stay out.

Sources

Based on "NPS Geodiversity Atlas — Apostle Islands National Lakeshore, Wisconsin," National Park Service, Geologic Resources Inventory, drawing on Thornberry-Ehrlich (2015) and Stucker (1974); a work of the United States government in the public domain.