In the Rocky Mountain region (fig. 1), extensive areas are underlain by geologic units that contain clays with high swelling potential. Beds or zones of high swelling clay occur in Cretaceous and Jurassic age marine and nonmarine deposits, shallow marine and continental Triassic rocks, Tertiary basin-fill deposits, and Quaternary alluvial, glacial, and lake deposits.

Geologic materials containing clays with slight to moderate swelling potential are widespread and range in age from Upper Paleozoic to Quaternary. This category includes extensive Tertiary basin-fill deposits, Quaternary lake bed sediments, and Jurassic and Triassic continental red beds and marine deposits.

Rocks with little or no swelling clay are mostly Precambrian and Paleozoic igneous, sedimentary, and metamorphic: rocks, and unaltered Tertiary volcanics and intrusives that form the major mountain masses of the region.

Cretaceous shale, claystone, and siltstone, with abundant high swelling days, are exposed or near the surface in wide areas of the Rocky Mountain region (Tourtelot, 1974). In general, Cretaceous sediments in the northern part of the region contain more smectitic mixed-layer clays and bentonite because parts of Idaho, Montana, and British Columbia were the scene of episodic volcanism throughout the Cretaceous Period. The farther the sedimentary deposit is from its volcanic source terrane, however, the greater the proportion of smectitic mixedlayer clays, because mixed-layer clays are of smaller particle size than other clays in the clay mineral suite (Gautier, 1983). In the Cretaceous units, smectites with high swelling potential may occur as the major constituent in discrete beds of "bentonite" (beds of altered volcanic ash), or more commonly as scattered or disseminated particles or debris (Schultz and others, 1980). The Pierre Shale in parts of Montana, Wyoming, Colorado, and New Mexico and the Bearpaw Shale in Montana are examples of widespread upper Cretaceous formations with highly swelling clays, capable of causing costly damage to homes, highways, and other man-made structures (Scott, 1969; Hogan, 1973; Sealy, 1973; Hart, 1974). Similarly, the Cretaceous Mancos Shale, extensive in parts of western Colorado, eastern Utah, and northwestern New Mexico, is known to cause construction and highway maintenance problems (Hepworth, 1965; Marchino, 197 1; Brakey, 1973; Price, 1973). The Tertiary and Cretaceous Denver Formation and upper Cretaceous Arapahoe and Dawson Formations in north-central Colorado are particularly well known for their swelling clay problems because of their occurrence in a metropolitan area where damage to homes and other light structures has been severe (Holtz, 1959; Sealy, 1973; Hart, 1974). Many Cretaceous units in Wyoming and Montana contain bentonites with high swelling potential. The Mowry, Belle Fourche, Carlile, Claggett, Niobrara, and Thermopolis Shales are noted for numerous bentonite beds ranging in thickness from a few inches to as much as several feet (Knechtel and Patterson, 1956, 1958).

Tertiary continental deposits with potentially high swelling clays include the intermontane basin deposits throughout southwestern Montana (Berg, 1969), the Fort Union Formation in eastern Montana (Smith and Redlinger, 1953; Berg, 1969; Hogan, 1973), and rocks of the White River Group in southeastern Wyoming and northeastern Colorado (Denson and Bergendahl, 1961; Scott, 1978). Deposits of bentonite or bentonitic material have also been reported in the Tertiary Uinta and Browns Park Formations of northeastern Utah and northwestern Colorado (Untermann and Untermann, 1964), and in lacustrine deposits of the upper Tertiary and Quaternary Santa Fe Group in central New Mexico (Patterson and Holmes, 1965).

Large quantities of potentially expansive clay also occur in deposits of Jurassic age. A study of clay minerals in the Morrison Formation in the Colorado Plateau areas of Utah, Colorado, and New Mexico by Keller (1962) revealed an "enormous" amount of smectite.

Clays with high swelling potential, largely derived from the alteration of volcanic debris, are abundant in parts of the Triassic Chinle Formation of southeastern Utah and northwestern New Mexico (Schultz, 1963; Molenaar, 1981). According to Patrick and Snethen (1976): "The shales of the Chinle represent some of the most expansive materials in the United States." Morris (1973) described severe highway maintenance problems in the Chinle Formation in an adjacent area of northeastern Arizona.

Quaternary deposits in the region, in general, have low swelling potential. In some cases, however, they may contain clays with high swelling potential, especially where derived from highly expansive parent materials. Pleistocene glacial till deposits in northern Montana (Colton and others, 1961) are an engineering concern where they are derived from expansive Cretaceous shales (Hogan, 1973). Varved glacial lake clays in west-central Montana are recognized as a potential cause of construction problems because of their high plasticity and shrink-swell capacity (Smith and Schuster, 1971; Lemke and Maughan, 1977). Other Quaternary deposits with local areas of potentially swelling clays include Pleistocene alluvial and loess deposits in north-central Colorado (Sealy, 1973; Hart, 1974) and alluvial and lacustrine deposits in east-central New Mexico where the Pliocene Ogallala Formation and eroded Permian rocks appear to be a major source of swelling clays (Glass and others, 1973). Some playa deposits in arid and semi-arid regions of western Utah and southeastern New Mexico reportedly contain prominent amounts of smectite (Guven and Kerr, 1966), and uplift has been observed near fissures of desiccation polygons on playa surfaces, suggesting a swelling clay effect (Neal and others, 1968).