This map and report summarize the geology of the northern part of the lower Verde River Valley in central Arizona. The map encompasses the Bartlett Dam 7.5' U.S.G.S. topographic quadrangle and the southern part of the Horseshoe Dam 7.5' US.G.S. topographic quadrangles (see Figure 1). The study area is on the northeast fringe of the Phoenix metropolitan area due east of Cave Creek and approximately 50 kilometers northeast of downtown Phoenix. The eastern side the study area includes Bartlett Dam, the western two-thirds of Bartlett Reservoir, and part of the Verde River. The center of the study area includes the volcanic hills of Brushy Mountain and overlying dissected basin-fill deposits. Late Tertiary basin-fill stratigraphy is well-exposed in the southeastern corner of the map. Most of the northern part of the study area is underlain by coarse-grained K-feldspar porphyritic granite which has been eroded into a well-developed pediment south of the drainage divide (see Figure 2).