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Generalized Geologic Map of the Conterminous United States
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  The North American Tapestry of Time and Terrain

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Ouachita Mountains

  map close-up showing the Ouachita Mountains and the western Appalachian MountainsSequences of folded Paleozoic rocks in the eastern Ouachita Mountains of Arkansas match those on the other side of the Mississippi River Embayment in the southern Appalachians of Alabama and Tennessee. Because these crustal wrinkles formed during the same period of continental-scale collision, the two ranges were once almost certainly continuous. The gap between mountain ranges, a 160-million-year-old structural trough, is a legacy from the time when the collided masses of North and South America parted ways and opened the Gulf of Mexico.