Paleontology and geology
Triassic rocks in Wyoming record the retreat of a shallow sea from the state. Early Triassic rocks are marine limestones and siltstones, while Late Triassic rocks include the easily recognized Chugwater and Spearfish Formations, which can be seen near Devil’s Tower in northeastern Wyoming, as well as in exposures in many other areas across the state. The bright red and orange of these siltstones and sandstones are caused by different amounts of iron oxides, and they were deposited in very nearshore marine environments and then on vast river floodplains as the sea regressed. The Nugget Sandstone, found in parts of western Wyoming, was deposited as eolian (wind-blown) sand dunes in beach or desert environments. Part of this sandstone is Jurassic in age.