Wyoming coal originated in two completely different types of peat swamps:

1. Those on the swampy margins of a shallow intercontinental sea
2. Those formed in intermountain valleys

Swampy Margins: The Late Cretaceous sea invaded the interior of the continent some 100 million years ago and remained for 40 million years. It expanded and contracted with the shoreline moving back and forth across Wyoming numerous times. Swamps formed on the exposed sea bottom at times of contraction (regression) only to be covered by sediment during periods of expansion (transgression). Finally, the Rocky Mountains began to rise causing the sea to retreat completely from the continent. The peat from these swamps formed the coals in most of western Wyoming.

Intermountain Valleys: Sixty five million years ago, as the Rocky Mountains began to rise, swamps formed in the valleys between the various mountain ranges. One such valley (far removed from the effects of the retreating intercontinental sea) was the Powder River Basin in NE Wyoming. These swamps were long lasting, producing extraordinary coal seams with single beds of 100 to 200 feet thick, underlying thousands of square miles. The coal is low in ash and extremely low in sulfur.

The climate was considerably warmer and wetter during both of these peat-forming periods. During the 'intermountain valley' time, the average temperature in Wyoming was 83° F and the rainfall 120 inches, compared with 42.5° F and 14.5 inches today. Palm trees were common. This fossil palm frond was found in a Rock Springs coalmine. Alligators and flamingoes were everywhere in Wyoming.