Sixty million years ago, the Powder River Basin did not exist for the nearest mountains were in Utah. Here was a flat, sea-level plain, muddy from a retreating sea that had covered Wyoming for 40,000,000 years. Sixty five million years ago, the land had begun to rise causing the sea to drain away.

Over the next 14 million years, the Basin rose to its present 3,000 feet, with the Big Horn Mts. and the Black Hills rising thousands of feet higher. The climate was subtropical averaging 80ºF with rainfall of 120 inches per year! Palm trees and crocodiles were common!

Due to the heavy rain and few rivers to carry the water away, the flat basin floor was a series of swamps and lakes for some 25 million years-the coal forming period. The swamps were so large that no sediment could get past the outside edges leaving the central portions free to accumulate pure peat. It was this clean peat that would eventually produce some of the thickest, low ash coals in the world.

The mountains rose in pulses around the basin. Each pulse buried the swamps with sediment. Eventually, the swamps would reform and last for thousands of years until the next mountain building pulse buried them again. Over time, hundreds of these peat deposits were stacked on top of each other, their thickness measuring the length of time a particular swamp existed before it was buried by another layer of sediment. The sediment (with its peat) turned into the rock and coal of the Fort Union and Wasatch Formations.

As the climate became drier and cooler, the swamps disappeared and the basin filled, burying the peat under thousands of feet of rock. This caused the temperature of the peat to rise changing it into coal. The last few million years has been a time of erosion of the basin down to where the coal is almost at the surface, perfect for open pit mining.