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Geologic Map
Shaded Relief
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Generalized Geologic Map of the Conterminous United States
North America Shaded Relief


  The North American Tapestry of Time and Terrain

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Legend and Rock Ages
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Glacial Limit


Map of  glacial limit

Late Cenozoic ice-cap glaciation and its many geomorphic effects mark a chapter in North America's geologic history that began some 2 million years ago and ended less than 20,000 years ago. The white line on the map is the southern boundary of the last of several ice-sheets that crawled south from Canada. These glaciers created changes in the texture of the land, particularly in the mid-continent, where topography north of the limit is muted and smooth, lacking the strong river-carved grain characteristic of unglaciated terrain to the south. Some features that owe their existence to glaciers include the Driftless Area, the Superior Upland, the Finger Lakes, and the Coteau des Prairies.