The El Rosario area is located 30° north by 115° 45' west along the Pacific coast of Baja California, Mexico. The geology of the area consist of Late Cretaceous marine and non-marine mudstones, silstones, sandstones, and conglomerates. Tephra deposits dated at approximately 77 m.y. and consisting of both air-fall and ash-flow tuffs are discontinuously exposed throughout a maximum area of 225 sq. km., but pre-dominantly concentrated within an 80 sq. km. sector adjacent to the coastline. These tephra deposits are contained within the El Gallo Formation, the lithology of which indicates a near shore lagoon and playa environment. The mechanics of the depositional basin have played a significant role in the preservation and subsequent lithology of the tephra units; most air-fall deposits being altered to sandy and silty quartzrich biotite tuff. The remaining exposures consist of vitric matrix ash-flow tuffs and relatively pure vitric air-fall tuff that is altered to olive green conchoidal bentonite. Two episodes of tephra deposition are recognized. The oldest is a relatively pure air-fall tuff that is bext exposed on Cerro Rayado, and a younger unit which is recognized by a thin brown ash-flow tuff which underlies white air-fall beds and is best exposed in a complete section at location 9. The tephra deposits are thickest and apparently more prevalent in the southwest portion of the area, suggesting a southern or western source. No volcanic centers are known to have existed in late creataceous time proximal to the El Rosario area. Three possible sources are inferred.