Fluvial and lacustrine-dominated clastic sedimentary rocks as thick as 1,800 m (6,000 ft) comprise the Paleocene Fort Union Formation and the Eocene Wasatch Formation of the western Powder River Basin in northeastern Wyoming and southeastern Montana. The systematic mineralogy of 45 samples of channel-fill sandstone from this sequence reflects the uplift and erosion of the Bighorn Mountains. Samples were collected to study vertical changes in the mineralogy of lower Tertiary sandstones adjacent to the Bighorn Mountains, lateral variations in the composition of the upper Paleocene Tongue River Member of the Fort Union Formation along the eastern front of the mountains, and variations in the composition of equivalent upper Paleocene sandstones of the central and western parts of the basin. Vertical changes in the mineralogy of a succession of Paleocene and Eocene sandstone units adjacent to the Bighorn Mountains most likely were produced by uplift and sequential erosion of the rocks that formerly overlaid the mountains. Uplift probably began in the middle Paleocene, during deposition of the Lebo Member of the Fort Union Formation, and continued into the Eocene. Differences in the mineralogy of the sandstone units along the western edge of the Powder River Basin that correspond to differences in the rock types now exposed along the crest of the Bighorn Mountains suggest that much of the erosional degradation of the Bighorn Mountains occurred during an early Tertiary tectonic episode. Lateral changes in the suite of unstable detrital grains within the Tongue River Member are compatible with facies and paleotransport studies that indicate a substantial eastward flux of detritus of early Tertiary age from the Bighorn Mountains into the central Powder River Basin.

Excerpt from Geology of the Bighorn Mountains Throughout the work assistance has been rendered by Mr. C. A. Fisher, who mapped portions of the area, measured numerous sections, collected fossils, and examined most of the crystalline rocks; and the author is indebted to Mr. Albert Johannsen for some of the petrographic descriptions. Previous observers have given but little information regarding the geology of the Bighorn Mountains, though Dr. F. V. Hayden ascertained the general relations of the uplift in his exploration of the Northwest, and Mr. George H. Eldridge, who crossed the range near Bald Mountain and southwest of Buffalo during the summer of 1893, described some of the broader features of the geologic succession and structure in a remarkably comprehensive manner, considering how small an area he had the opportunity to examine. The Bighorn region is very thinly settled, there being no permanent habitations among the mountains, and it is but little visited by others than hunters, prospectors, and herders. There have recently been established two summer resorts in the mountains, and each year a larger number of persons visit the region. Unfortunately the mineral prospects have proved disappointing to the prospectors, and there appears to be but little promise that the area will become important on account of its mineral resources. In the Bridger uplift promising prospects of gold and copper have recently been discovered, which may prove of value. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.