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TEACH Shoreline Geology

5 | Isle Royale

Located in the northwestern section of Lake Superior, the archipelago of Isle Royale National Park is a reminder of what primitive America's landscape looked like. Both molten lava and glaciers shaped the shorelines and inland lands, and created a landscape unique to Isle Royale.

The Palisades. Click for larger image. Over one billion years ago, the earth's crust cracked in what is now the area of Lake Superior, and molten lava poured out onto the land. Exploding over 100 times, lava spouted from the cracks in the earth, coating the earth with layer upon layer of lava; the weight of the lava eventually sunk the land and formed the Lake Superior basin. Some of the lava flows in this region, like the Greenstone Flow, are among the largest and thickest flows in the world. They took tens of hundreds of years to cool and solidify, and in doing so formed the giant columns of the Palisades.

Red Rock beach. Click for larger image. Isle Royale's southern and northern shorelines differ greatly. About 11,000 years ago, the last glacier starting retreating from Isle Royale. There was a pause in the glacial retreat when the glacial ice front lay across the southwest end of Isle Royale; after that long pause, the ice rapidly retreated across the rest of the island, leaving a thin mantle of deposits at the southwest end, but very little material on the central and northeast sections, where the ice melted quickly.Bluffs on Isle Royale. Click for larger image. Today, you can see the result of this glacial retreat by comparing the beaches of the south shore (photo above) with the north shore (photo at right). The southernmost beaches are composed of reddish sedimentary rocks deposited during the long pause of the last glacier, while the northernmost tip of Isle Royale is composed of rocky bluffs.

Graphics: The Palisades; red rock beach on Isle Royale's southern shoreline; Isle Royale's northern shoreline

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