In 2009 the University of Utah Press and the Utah State Historical Society co-published three volumes of long out-of-print journals, letters, and other documents from John Wesley Powell"s expeditions down the Colorado River. We are proud to announce the fourth and final volume. Cleaving an Unknown World collects Powell"s journal (Smithsonian Journal of History, 1968); Jack Hillers"s diary and photographs, previously published as Photographed All the Best Scenery, edited by Don D. Fowler (University of Utah Press, 1972); original maps from Francis Marion Bishop (Utah Historical Quarterly, 1969); Frederick S. Dellenbaugh"s letters (Utah Historical Quarterly, 1969); and John C. Sumner"s journal from the first Powell expedition (Utah Historical Quarterly, 1969). Roy Webb"s foreword provides the context for these disparate pieces. This beautifully illustrated book features Hillers"s photographs?long regarded as a remarkable and unique record of the Colorado River and the Grand Canyon. Cleaving an Unknown World belongs in the library of any reader interested in the exploration of the American West. Â