The cutaway profile shows layers of rock color-coded by age, the youngest lying in the valleys where the Bighorn and Powder Rivers run, and the oldest peaking against the sky, ancient roots down into deepest earth. It gives an approximate range in long-ago years of geological and evolutionary time, and shows what was happening to life during the billions of years of this immensely long period. Our era Age of Mammals, Cenozoic-Tertiary. Rockies formed Mesozoic, Triassic. Appalachians formed Age of Fishes, Paleo-Devonian.
Algae create oxygen atmosphere.
Take another look at the way the epochs of rock-time layer over that ancient peak. The other continental roots don't show this neat reverse epochal order of layers, or even layers for all epochs. The other continental nuclei are elevated outcroppings of the oldest Pre-Cambrian rocks, interspersed in jumbled fashion with newer ones. Along the range's spine, this character of most ancient age exposed to the sky is not maintained very far north or south of the Wheel, although the physical appearance of the mountains is the same. Thus the Medicine Wheel floats on a high island of time, the world's oldest rocks, layered in reverse order of the epochs of geological history.