The Morrison Formation is predominantly claystone with interbedded sandstone, siltstone, and thin carbonate units. The maximum thickness is 350 feet. It lies conformably and unconformably upon different units and underlies unconformably the Purgatoire Formation of Cretaceous age. Most of the sandstones are quartzarenites or subarkoses. Calcite and microcrystalline quartz are the cements. Sedimentary rocks were predominant in the source area, and acid igneous and metamorphic rocks contributed minor amounts of detritus. Illite, the most abundant clay, probably came from illite-rich sedimentary rocks; montmorillonite, the predominant clay mineral in the middle Morrison, was formed from altered volcanic ash. Sediments accumulated on floodplains formed by numerous small meandering streams. Deposition of carbonates was in small lakes on low areas of the floodplains.