The Parkman Sandstone Member of the Mesaverde Formation (Upper Cretaceous) in the southwest part of the Powder River Basin consists of a sequence of sandstone, siltstone, and shale, including both marine and nonmarine strata. The Parkman represents a rather typical regressive sequence, grading upward from the offshore marine siltstone of the Steele Shale to nearshore marine and beach sandstones in the lower sequence of the Parkman. This is overlain by nonmarine shale, siltstone, and sandstone in the middle sequence, representing deposition in floodplain, fluvial, and swamp environments. The upper Parkman consists of nonmarine sandstone at the base to possibly marine sandstone at the top. Thickness of the Parkman ranges up to 480 feet. Facies in the area are complex as a result of intertonguing of units due to minor oscillations of the sea. There is a distinct facies change in the lower sequence from the series of interbedded sandstones and siltstones in the east to the essentially massive sandstone in the west. During deposition the strandline probably trended in a north to northwest direction in the area, with marine deposition to the east and nonmarine deposition to the west. Facies changes in the middle and upper sequences are less pronounced in the area and are difficult to trace. Stratification is the most conspicuous primary structure in the Parkman. The units are generally even-bedded. Cross-stratification of various types and scales with generally low to moderate angles of inclination of the laminae, aids in interpreting particular environments. Large scale contorted stratification is conspicuous in the lower sequence of the Parkman and is probably a result of deformation of nearshore marine sediments. Fossils are relatively sparse in the Parkman in the area of study. Ophiomorpha in the lower sequence is indicative of shallow neritic to lower littoral environments. Marine and fresh water pelecypods are present in the lower and upper sequences respectively, and plant remains and reptile bone fragments are found in the middle sequence. Lack of fossils causes some difficulty in making environmental interpretations and regional correlations.