This is a remarkable edition of Warren’s map that has been amended to show geological features provided by Dr. Ferdinand Hayden, then Professor of Geology and Mineralogy at the University of Pennsylvania. It illustrates the geology of the northwestern Great Plains region extending from the Canadian border south to Pueblo, Colorado, and from the Colorado Front Range east to Leavenworth, Kansas. The map itself provides a comprehensive view of the surveys and exploration of the region, including those of the Government Land Office, Pacific Railroad Surveys, Stansbury, Fremont, Nicollet and others, It is one of Hayden’s earliest and most distinctive large geological maps with the Black Hills shown as a brightly colorful "bull's eye," surrounded by pastel shaded tertiary formations. The map also provides excellent detail of railroads, including the Union Pacific, and dozens of forts and Indian ranges. The color key to the right of the title identifies eight different geological types. The map was published in the Preliminary Report of Explorations in Nebraska and Dakota, in the Years 1855-56-57, reprinted by the Government Printing Office in 1875.

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Condition: B

Issued folding with scattered foxing, light offsetting, and numerous short splits at the fold junctions. A few short tears and a 3.5" fold separation at top have all been closed on verso with archival tape.