Of all the elements that find their way into the coal, the one of major concern is sulfur. Upon combustion of coal, the sulfur is released as various oxides which will combine with moisture in the atmosphere to produce sulfuric acid (one cause of acid rain).

Sulfur arrives in the peat either as part of the original wood or more commonly, as sulfates from adjacent seas (storm floods or coastal rains). The sulfur then undergoes a series of transformations involving oxidation and reduction reactions and aerobic and anerobic bacterial decay to form pyrite (iron sulfide).

Wyoming coal is noted for its extremely low sulfur content (0.4% to 0.06 % with Eastern coal at 3.0% to 5.0 % or more).

While many of Wyoming's early coal swamps (Cretaceous Period) were, like the Eastern US coal swamps, adjacent to an inland sea, they were on the western shore so the prevailing westerly winds blew the salty rains out to sea instead of onto the swamps. These are the coal beds of Rock Springs and Kemmerer in western Wyoming.

The later coal swamps (Paleocene and Eocene Periods) were in isolated mountain valleys far removed from any influence of the sea. These coals are in the Hanna and Powder River Basin coal fields.

Here, sulfur is accumulating at the surface having been carried up by the gasses from a burning sulfur rich coal seam below.