The Picacho Mountains are north-south trending mountain range completely surrounded by Quaternary alluvium, and consists of Tertiary and older granitic and gneissic rocks (Figure 1). Picacho Peak, located south of the south end of the range, is also surrounded by alluvium, and consists of Tertiary andesitic volcanic rocks. Picacho Peak and the Picacho Mountains are separated by a gap of shallowly buried bedrock through which pass Interstate 10, the Southern Pacific Railroad, and the Central Arizona Project canal. The Picacho Mountains consists of a compositionally diverse suite of Tertiary, Cretaceous or Proterozoic granitoids, heterogeneous to gneissic granite, muscovite granite, schist, and gneiss, much of which has been affected by middle Tertiary mylonitic deformation and probably by late Cretaceous synplutonic deformation [Rehrig, 1986]. Mylonitization is inferred to have accompanied normal faulting and ascent of the bedrock from mid-crustal depths in the footwall of a moderate to low-angle normal fault commonly known as a "detachment fault". Older gneissic fabrics may record Laramide mid-crustal deformation, or relict Proterozoic fabric. The crystalline rocks of the Picacho Mountains are part of the footwall of a south- to southwest-dipping detachment fault that is exposed in only one small area in the southern Picacho Mountains. Picacho Peak, which consists almost entirely of northeast-dipping basaltic and andesite volcanic rocks, is part of the hanging wall of the detachment fault. Geologic map of the Picacho Mountains and Picacho Peak, Pinal County, southern Arizona - report and two map sheets, scale 1:24,000.