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The northern Rocky Mountain region (Figs. 1, 2), contains a relatively complete Phanerozoic stratigraphic section ranging in age from Cambrian to Holocene (Figs. 3 to 5). Thicknesses of Paleozoic rocks are as much as 9,000 m in southeast Idaho, 3,000 m in southwest Montana, 2,500 m in the central part of the Williston Basin, and generally less than 1,500 m in most of Wyoming, South Dakota, and the remainder of Montana. Thicknesses of Mesozoic rocks are as much as 11,000 m in southeast Idaho and western Wyoming, 6,000 m in southwest Montana, and generally less than 3,000 m in northern and eastern Wyoming, South Dakota, and central and eastern Montana. Thicknesses of Tertiary rocks are as much as 300 m in the western part of the Williston Basin, more than 900 m in the deeper parts of the Powder River Basin, and as much as 3,000 m or more in the Tertiary basins of western Montana and southwestern Wyoming.

Cambrian strata unconformably overlie Proterozoic sedimentary rocks in western Montana, east-central and southeastern Idaho, and overlie older Precambrian metamorphic and igneous rocks in the remainder of the region. Thicknesses are more than 1,500 m in south-central Idaho and more than 900 m in western Montana, thinning relatively uniformly eastward and pinching out along the flank of the Transcontinental Arch (Figs. 3 to 6). Cambrian rocks comprise a sequence of marine sandstone, shale, and limestone, which represent the shelf facies of a broad eastward transgression of the Cambrian Cordilleran sea across the

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