Northeastern Wyoming is occupied by a broad structural basin opening to the north and bounded on the east, south, and west by three mountain uplifts - the Black Hills, the Laramie Mountains, and the Big Horn Mountains. (See fig. 16.) Throughout much of this basin the surface rocks are of Tertiary age. They contain the principal coal beds of the region and have been classified in all the most recent Survey reports as Fort Union. The true age of these rocks and the general stratigraphy of the fresh-water beds that underlie them are the theme of this paper.
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Wasatch fossils in so-called Fort Union beds of the Powder River Basin, Wyoming, and their bearing on the stratigraphy of the region