Marine sedimentation on the Wyoming Cratonic Shelf in Pennsylvanian time left its record in the transgressive Sacajawea shale, the "still-stand" Amsden carbonate rock, and the regressive Tensleep sandstone. Primary source of sand was the Precambrian Canadian Shield, 80 percent being well-rounded grains from sedimentary rocks north and northeast of Wyoming. Underlying Madison limestone having a karst surface was the probable source of the red clay for the Sacajawea formation. The dolomitic Amsden formation marks the period of greatest inundation. Crossbedding in the Tensleep sandstone suggests a bathymetric slope southwestward. Feldspar in all three formations gives a clue to the changing climatic environment.

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