Purpose and Method: One of the most complete sections of Upper Precambrian through Middle Cambrian rocks in northwestern Sonora is present in the Sierra de la Berruga. This study was undertaken to describe the stratigraphy and internal structure of the range, establish easily recognized cartographic units in the thick Precambrian sequence, and determine the depositional environment. Approximately thirty-one days were spent mapping and measuring stratigraphic sections between December, 1970 and January, 1972. Mapping was done on aerial photographs with a nominal scale of 1:67,000. Because an adequate topographic map of the area does not exist, a drainage base map was prepared. Throughout hilly areas, only central portions of the photographs were used in order to minimize effects of radial displacements. The resultant drainage map is a traced composite made from thirteen photographs. Maximum mismatches of drainage lines from one photo to the next was less than two millimeters. Approximately seventy thin-sections were studied to determine the allochemical composition and depositional environment of the carbonate rocks, the provenance of the terrigenous sedimentary rocks, and general rock descriptions.