This chapter summarizes the results of the writer’s work on the paleogeography and facies changes of the Monterrey Formation in the San Hilario area of Baja California, as part of the exploration for phosphate rock prospects carried out in the area by the Mineral Resources Council (Consejo de Recursos Minerales).
Field work from March 1975 to January 1976 covered detailed mapping on l:10,000-scale vertical airphotos of the Monterrey Formation outcrop belt, an area 70 km long and 4 to 5 km wide, together with the upper and lower portions of the respectively under- and overlying Tepetates and San Isidro Formations.
Additional information includes six cross sections along the principal stream beds, eight lithologic columns (scale 1:100), and a stratigraphic correlation table of these columns (1:2,000). As an aid to correlation and future detailed geological work, a collection of Polaroid photographs describes virtually all the Monterrey outcrops exposed at the streambeds, focusing on the section’s geoeconomic features. Part of this material is included here.