Geologic Formations

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The Apostle Island archipelago's sandstones were deposited during the late Precambrian era, about one billion years ago, and form the basement rock for all the islands. The upper and lower most layers (Chequamegon and Orienta formations) are in the Precambrian Bayfield Group and were deposited by northeastward-flowing braided streams. The Devils Island Formation, between the sandstones, represents deposition across sand-flats that were intermittently covered by shallow ponded water. The Pleistocene ice advances provided an abundance of till, with lesser amounts of glacial outwash, which covers most of the island. Some glacial drift was streamlined by overriding ice. Terraces, wave-cut benches, and elevated beaches show evidence of higher levels of Lake Superior. High bluffs and glacial drift erode to provide sand for today's sandspits and cuspate forelands. The Apostles were glaciated repeatedly, with the most recent glaciation was by the Vanders Ice Lobe approximately 12,000 years ago. In the wake of this ice lobe melt waters varied drastically in elevation and longevity. Glacial Lake Duluth submerged most of the Bayfield Peninsula and the islands. Continued and rapid deglaciation opened up new drainages and with land rebound, it lowered the lake level making the Apostles part of the Bayfield Peninsula. Following these extreme periods of rising lake levels controlled by land rebound, the Apostles were separated from the peninsula to form the archipelago. Above the bedrock there are regionally rare habitats that exist in the lakeshore. These habitats include old growth forests, clay bluff communities, sandstone communities, lagoonal and bog communities, and dunal communities. The Wisconsin Department of Natural Resources Natural Heritage Inventory Program has designated Maritime Forest, Sandscape (includes beaches, sandspits, cuspate forelands, and tombolos), and Maritime Cliff State Natural Areas within the Lakeshore.



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