Clay minerals are some of the most interesting and variable aspects of the lithology of the Morrison Formation. Clays have been utilized for many diverse applications, including paleoenvironmental analyses and temporal correlations. The chemical nature of clay minerals, however, allows them to be easily influenced by environmental conditions, both immediately after deposition and over the millions of years of burial. This study looks at the clay mineralogy of the Morrison Formation using X-ray diffraction analysis, and concludes that:(1) the clay mineralogy of the Morrison Formation is highly variable, both laterally and vertically; and (2) there does not appear to be a consistently developed horizon of clay change from illitic to smectitic clays. As a result of these findings, caution is urged in using clay minerals for correlative purposes or as paleoenvironmental indicators.