In west-central Wyoming the Red Peak Member (Triassic) of the Chugwater Formation is predominantly composed of red and drab, silty, very fine-grained sandstone and well-sorted and poorly-sorted siltstone. If the sandstones and siltstones are classified according to sandstone nomenclature, they are arkoses, subarkoses and impure arkoses. The average well-sorted siltstone contains smaller amounts of quartz, quartzite and chert, and larger amounts of "coarse" mica, opaque minerals, rock fragments and authigenic carbonate than the average very fine-grained sandstone. Compared with well-sorted siltstone, the average poorly-sorted siltstone contains smaller amounts of authigenic carbonate and larger amounts of "coarse" mica and matrix. Plagioclase:potassium feldspar ratios from siltstones and silty claystones are nearly three times larger on the east than they are on the west, based on X-ray data. There is a general westward increase in the amount of potassium feldspar, which is probably either the result of a size bias or the reflection of different or new source areas. Quartz:feldspar ratios are about the same at each locality, suggesting that there is little difference in the maturity of the siltstones and silty claystones. Quartz: feldspar ratios calculated from the modal analyses are close to the ratios for graywacke and arkosic sandstones, indicating that the rocks are generally immature. The Al 2 O 2 :Na 2 O ratios of four siltstones and very fine-grained sandstones are close to the Al 2 O 3 :Na 2 O ratios for graywacke and arkosic sandstones. Based on X-ray data, there is a northwestward increase in percentage of dolomite cement. The carbonate relations probably indicate that waters were less saline on the east and southeast, and more concentrated on the west and northwest. Increased gypsum on the west partly confirms this interpretation. The clastics were mainly derived from sedimentary, acid plutonic and gneiss-schist terrains from source areas on the north, east and southeast. The climate of the depositional sites and the source areas is inferred to have been semi-arid to arid.