The westward sweep of gray to blue represents sedimentary rocks of Pennsylvanian to Permian age, most of them laid down in shallow seas. In the northeast is part of the uplifted Ozark Plateau, which preserves older rocks of Mississippian down to Devonian age.
The strip of green in southernmost Oklahoma represents Cretaceous-age rocks from a later incursion of the sea. And in the western panhandle are still younger layers of rock debris that were shed from the rising Rockies in Tertiary time, after 50 million years ago. These have been eroded in more recent time to reveal deep-seated older rocks in the farthest west end of the state in the High Plains.
Learn much more about Oklahoma's geology at the Oklahoma Geological Survey site.
More Ohio resources on About.com:
Oklahoma Geography, State Symbols & Facts
Oklahoma National Parks
Oklahoma State Parks for Spring
Oklahoma State Parks for Winter