The stratigraphy of the Pierre shale, basal formation of the Upper Cretaceous Montana group in Colorado, is discussed in detail. Measured sections along the Front Range are described, and correlations of these sections with others in central and western Colorado are presented by means of three-dimensional diagrams. On the basis of lithologic and faunal characteristics the Pierre of northeastern Colorado is subdivided into four zones, from the base upward: the Sharon Springs shale zone, the Rusty zone, the Hygiene zone, and the Transition zone. Five units are locally recognized in the Hygiene zone, and three of these are present in much of Colorado. The Pierre zones are traced throughout eastern and central Colorado, and tentative correlations are suggested with the Mancos and Mesaverde formations of western Colorado.

The problem of the origin of the Hygiene sandstones is discussed. Regional lithologic studies indicate the possibility of a local origin of this sandstone tongue.