Where glaciers did not tread; ice now lodges in crevices, creating miniature ice age habitats in North America's Driftless Area.(Driftless Area National Wildlife Refuge)

Publication: Natural History

Publication Date: 01-OCT-05

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COPYRIGHT 2005 Natural History Magazine, Inc.

During the Pleistocene epoch, between about 1.8 million and 10,000 years ago, a series of ice ages swept over the Earth. The glaciers of each ice age covered large regions of the planet, particularly in the Northern Hemisphere. The geological traces of their comings and goings are easy to spot, in scoured bedrock, in large isolated boulders (known as glacial erratics), and in deposits of gravel, sand, silt, and clay (known as glacial drift). The two most recent ice ages, known in North America as the Illinoian (between 170,000 and 120,000 years ago) and the Wisconsinan (between 70,000 and 10,000 years ago), were quite severe at their maximums. Somehow, though, their glaciers bypassed a 15,000-square-mile area of what is now southwestern Wisconsin, southeastern Minnesota, northwestern Illinois, and northeastern Iowa, even though earlier Pleistocene glaciers had covered the region. Because early geologists did not find recent glacial drift in the region, it became known as the Driftless Area.

With precipitous limestone cliffs, deeply shaded ravines, and clear rocky streams, the...

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