S uites of bentonite beds in the Mowry Formation and in the lower part of the Frontier Formation in a 40,000-square-mile area of north-central Wyoming were sampled and described at measured sections in the interval between the Muddy Sandstone and the sandstone in the lower part of the Frontier Formation. After binocular examination, more than 800 samples were studied by X-ray and microscopic methods. The sand fraction yields a typical igneous mineral assemblage. Montmorillonite, the chief constituent of bentonites, is relatively uniform except for significant differences in the exchangeable cations. The chemical composition of the montmorillonite seems to be related to the amounts of the various types of feldspars. By combining mineralogical composition with subsurface data from electric logs, the bentonites can be correlated broadly. Most of the beds consist of coalescing lobate forms as shown by isopach maps. Based on areal correlation and lithologic descriptions of the bentonites, the stratigraphic relationships of the Mowry Formation become evident. Bentonites have the same physical origin as recent wind-transported volcanic-ash beds. Mineralogy, textures, and distributions of the Wyoming bentonites are also similar to recently deposited ash beds. The gross and a few detailed features of bentonites and ash beds can be explained by a qualitative physical model. The important factors which determine the physical properties of the ash beds are intensity of volcanic explosion, amounts of gaseous and solid materials, particle-size range of the solid materials, and the high-altitude winds (above 40,000 feet) which transport the ash to depositional sites. The igneous source-rock type of the bentonites may be determined fairly accurately from analyses of the sand-sized fraction. Analyses of the Wyoming bentonites suggest source-rock variations from rhyolite to andesite, although dacite, latite, and quartz latite are the common types. Geological evidence indicates that the vitric ashes of the Wyoming bentonites altered after deposition probably by hydrolysis. The process may be expressed as a series of acid-base reactions. The minerals formed by alteration indicate alkaline conditions, but the high magnesium content does not preclude a neutral to slightly acid environment. The stratigraphy of the bentonites suggests an intimate relationship to the sedimentational and tectonic history of the Cretaceous beds. Major bentonites invariably occur in the same cyclical sequence—ideally, from bottom to top, shale, coal, bentonite, shale grading to sandstone or conglomerate. If the bentonites are used as time lines, a tectonic interpretation of sedimentational patterns is possible. The history is as follows: (1) high basin, (2) volcanic activity (to the west), (3) uplift in the source and coincident subsidence in the basin, (4) continuation and intensification of (3), and (5) stand-still or abrupt uplift in basin with cessation of uplift in the source. We propose three major orogenic pulses in the Cretaceous as represented by Thermopolis-Mowry-Frontier, Cody-Mesaverde, Lewis (and Meteetse)-Lance Formations. Minor orogenic pulses represented by intraformational cycles are superimposed on the major events.