The field work of which this report is a record was done in the summer and fall of 1921 by members of the United States Geological Survey. A project to build a large storage dam at Lees Ferry, on Colorado River in northern Arizona, called for a detailed topographic survey of the area covered by the project, for the purpose of determining the capacity of the reservoir. This work was undertaken by the United States Geological Survey in cooperation with the Southern California Edison Co. Three surveying parties were sent to the field, each accompanied by a geologist, whose specific duty was to study and report on the rock formations within the area to be flooded. One topographic party, under A. T. Fowler, which started at Lees Ferry and worked up stream in Arizona, was accompanied by Kirk Bryan. Another party, under K. W. Trimble, which started near Bluff and worked down the San Juan and thence down the Colorado, was accompanied by H. D. Miser. The third party, under W. R. Chenoweth, worked from Fremont River to the Waterpocket Fold and then returned to Green River, Utah, and traversed Cataract Canyon during the period of low water. C. R. Longwell was with this party until September, when his place was taken by Sidney Paige. Mr. Paige, in company with the Kolb brothers, E. C. La Rue, and Henry Ranch, left the Chenoweth party after Cataract Canyon had been surveyed and rowed down the Colorado to the mouth of the San Juan, where they were joined by Mr. Miser. Then they took a hurried trip by boat down the Colorado to Lees Ferry, making a few short stops and visiting the famous Rainbow Bridge. Thus the geology of the canyons of Colorado and San Juan rivers and of the lower parts of tributary canyons was examined continuously, and reconnaissance work was done in the country back from the rivers. At the same time a fourth party, under R. C. Moore, was mapping parts of Kane, Garfield, and Wayne counties, Utah, to determine whether oil might be found there. The present paper includes brief descriptions of the rocks of the regions traversed, detailed geologic sections, and columnar sections measured not only by the geologists who accompanied these parties but by other geologists who have worked in the same regions or in adjoining regions. The positions of the columnar sections measured and many of the other sections are shown on Figure 1.