The coal beds of the Powder River Basin, Wyoming are famous for their extreme thickness, often exceeding 100 feet, with one reaching 280 feet (five and ten foot seams are considered good in other parts of the country). Such thick coals require that the swamp conditions remain constant for thousands of years. The swamps must be composed primarily of trees, be stagnant, develop high acidity (low pH) at depth, and be protected from sediment being brought in by rivers.

While there are hundreds of coal seams in the Powder River Basin, currently al but one of the larger mines are producing out of a single coal bed that covers thousands of square miles and averages 100 feet thick (The Wyodak Seam).

The fact that we have such thick coal seams tell us these conditions must have been met:

1. It was a freshwater wetland. Thick coals are made of peat from woody plants which are only found on land. Marine environments have too high a pH (8) to preserve peat.



2. The wetland was a swamp. Microscopic analysis of coals reveals them to be made primarily of the remains of woody tissue so the wetland was primarily a swamp (wetland with trees) as opposed to a marsh (grasses) or a bog (mosses).



3. Stagnant water. The wetland must have had stagnant water for three reasons:



1. Currents could bring in oxygenated water allowing too much decay.



2. The acid produced by decay must remain in order to lower the pH enough so that decay will be stopped, preserving a portion of the peat.



3. Currents may remove some of the accumulating peat.



4. Peat stays submerged. The partially decayed peat must remain under water (stagnant, acidic water) to prevent complete oxidation and loss. Exposure to the atmosphere will quickly oxidize some of the peat. A few months of a dry period, however, will not matter. A small amount of peat will oxidize and upon later flooding and more peat accumulation, will show up as a layer of charcoal (the maceral Fusain) in the coal.



5. Accommodation space. The basin must have subsided continuously to provide space for the necessary amount of peat to accumulate. Alternatively, the water table could slowly rise providing the accommodation space.



6. Rate of subsidence. The rate of subsidence must be just right: too fast and the swamp will flood producing a lake, too slow and the swamp will become dry land.



7. Long period of constant conditions. Since peat accumulates at the rate of approximately 1 millimeter per year, it takes thousands of years to produce a thick coal seam.



8. Optimum climate. Durring the formation of the Wyodak peat, the average temperature in Wyoming was 80°F with 120 inches of rainfall!