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Geologic Map
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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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Strange Grain

magnified area of unusual grain with arrow showing direction A north-northwest trending terrain texture, or grain, evident as myriad aligned ridges and valleys carved by small streams, pervades the northern and central Great Plains. This widely distributed pattern disappears north and east of the Pleistocene glacial margin. Both regional tectonism (fine-scale fracturing in the Earth's crust created by opposing horizontal forces) and surface processes (late-glacial wind) have been suggested for the origin of this widespread texture.
Close-up of
 upper great plains region, with box outlining area of opposite grain