The trace of the Martinez Ranch fault, which delineates the steep eastern flank of the Rincon Mountain~, was mapped northward for 30 kIn from the 1-10 freeway near the hamlet of Mescal to the eastern shoulder of Mica Mountain at the head of Deer Creek. Mapping was extended eastward to include the Happy Valley basin of Neogene gravels, and tiltblocks of an extensional allochthon exposed beneath Happy Valley basin fill where the allochthon lies structurally above the San Pedro detachment fault capping exposures of the mylonitic lower plate of the Catalina core complex. The field mapping, at larger scale (1:24,000) than other mapping published to date, was undertaken to understand better the influence of post-detachment normal faulting on the configuration and morphology of the Catalina core complex (Dickinson, 1991). The legend for the accompanying geologic map is appended to the text (p. 16).