The subject matter of this bulletin is based chiefly on the results of a systematic program of geological investigations in Yellowstone National Park, conducted by the U.S. Geological Survey during the years 1965-71. The program, ably organized and directed by A. B. Campbell, required the special skills and knowledge of many individuals to make a comprehensive study of all the varied and complex features of the Park area. Without their invaluable cooperation, assistance, and interest, this endeavor to summarize the geologic story of Yellowstone would not have been possible. I therefore express my sincere thanks to the colleagues listed below, all of whom furnished unpublished information bearing on different aspects of that story: R. L. Christiansen and H. R. Blank, Jr. (Quaternary volcanism); H. W. Smedes and H. J. Prostka (Absaroka volcanism); D. E. White, L. J. P. Muffler, R. O. Fournier, and A. H. Truesdell (thermal activity); G. M. Richmond, K. L. Pierce, and H. A. Waldrop (glaciation); E. T. Ruppel and J. D. Love (sedimentary rocks and geologic structure); J. D. Obradovich and Meyer Rubin (radiometric dating). W. L. Newman provided many helpful suggestions regarding the preparation of the manuscript.

The geological studies in Yellowstone received the full support and cooperation of former Park Superintendent J. S. McLaughlin, Superintendent J. K. Anderson, and other personnel of the U.S. National Park Service. In particular, the helpful advice, interest, and enthusiasm of the entire Park Naturalist staff, especially J. M. Good and W. W. Dunmire, former and present Chief Park Naturalists, respectively, greatly facilitated the work in all phases of the program.