When, in 1915, there was brought to light on the Mesa Verde National Park, Colorado, the mysterious structure, Sun Temple, the author recalled well-known descriptions of towers and other related buildings that have been recorded from other localities in southwestern Colorado and Utah. The published descriptions of these structures did not seem to him adequate for comparisons, and he planned an examination of these great houses and towers, hoping to gather new data that would shed some light on his interpretation of Sun Temple. During the field work in 1917, thanks to an allotment from the Bureau of American Ethnology for that purpose, he undertook a reconnoissance in the McElmo district, where similar buildings are found and where he believed cultural relatives of the former inhabitants of Mesa Verde once lived. In 1918 he extended his field work still farther. He investigated ruins as far as the western tributaries of the Yellow Jacket Canyon, penetrating a short distance beyond the Colorado border into Utah. The object of the following pages is to make known the more important results of this visit, and interpret the evidence they present as a contribution to our knowledge of the extension in prehistoric times of the Mesa Verde culture area. HISTORICAL Attention was first publicly called, about 40 years ago (1875-1877), by Messrs. Jackson,[1] Holmes, Morgan, and others, to some of the ruins here considered. It is difficult to identify all of the ruins mentioned or described by these pioneers. Their “Hovenweep Castle” is supposed to lie in about the center of the district here considered, possibly on Square Tower (Ruin) Canyon, although the large castellated building[2] in Holly Canyon would also fulfill conditions equally well. Their “Pueblo” may have been situated on the McElmo near the mouth of Yellow Jacket Canyon. Early writers rather vaguely refer to a cluster of castles and towers as situated some distance from the “Burial Place,” which is readily identified on the promontory at the mouth of the McElmo, as probably those in Square Tower (Ruin) Canyon, but the cluster may be either at Square Tower or Holly Canyon, both of which are about the same distance from this site. As “Pueblo” is not indicated on the map accompanying the Hayden report, the sites of rock shelters “some 7 miles from ‘Pueblo’ and 3 miles from the McElmo” remain doubtful. The author retains the name “Hovenweep Castle” for the ruin in Square Tower Canyon. [1] Ancient Ruins in Southwestern Colorado. Rept. U.S. Geol. Surv. Terr. (Hayden Survey) for 1874, Washington, 1876. [2] The situation of a spring near Hovenweep Castle indicates that the Great House may be the Hovenweep Castle of early writers.