In pioneer days the “Overland Trail” crossed the North Platte River in southeastern Wyoming, about one mile from the scene depicted above. Cottony cumulus clouds drift over the distant, snow-covered heights of the Medicine Bow Mountains. The red sedimentary rocks (Mesa Verde formation) contain the remains of dinosaurs, the whitish slopes (center) are deposits of volcanic ash, and the greenish rocks (left) are intrusives forced into the sediments when below the surface of the earth.