NPS photo/G M Spoto Explore DeeperAncient Sands, Ice, and WaterThe Apostle Island Archipelago and Bayfield Peninsula’s sandstones were deposited during the late Precambrian era, from almost a billion years ago until about 660 million years ago. These sandstones, the Bayfield Group, 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, sandwiched between the other members, is the product of a depositional environment of sand-flats, intermittently covered by shallow ponded water, resulting in thinner, more easily eroded layers perfect for sea cave formation. The Pleistocene ice pulses sculpted the islands and provided an abundance of till, and some glacial outwash, that covers most of the islands. Terraces, wave-cut benches, and elevated beaches show evidence of higher levels of Lake Superior. High bluffs of glacial material erode to provide sand for coastal seascapes of sandspits, beaches, tombolos and cuspate forelands (almost a spit but too stubby). The Apostles were glaciated repeatedly, most recently by the Vanders Ice Lobe about 12,000 years ago. In the wake of this ice lobe retreat, melt waters trapped between highlands and a wall of ice, varied drastically in elevation and longevity. Glacial Lake Duluth submerged most of the Bayfield Peninsula and the islands. Deglaciation and rising land from the removal of the weight of ice, created new drainages lowering lake levels exposing the Apostle Islands and more of the Bayfield Peninsula. GeologyShowing results 1-3 of 3
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Last updated: March 7, 2022