The area east of Rosarito Beach contains a sequence of rocks of Early Cretaceous to Pleistocene age. Slightly-metamorphosed volcanic rocks of the Alisitos Formation form the basement in the area. Late Cretaceous mudstone and sandstone of the Rosario Formation lie with angular discordance on the Alisitos Formation. Conglomerates of Formation A disconformably overlie the Rosario, and lie with angular discordance on the Alisitos. Sandstone and basalt of Formation B lie with angular discordance on the Alisitos and disconformably overlie tbe·, Ro'sario Formation and Formation A. Post-Formation B deformation has produced what appears to be an anticline with its axis trending north-westerly. The crest e ·, t KITI,of the fold is about tt'o"e R+i-lee east of Rosarito Beach near the eastern border of the map area. The fold is defined in sandstone of Formation B by westerly dips in the western part of the mapped area and easterly dips in the eastern part. The underlying Rosario Formation dips to the west on the western side of the anticlinal axis, and is not exposed or was not deposited in the area east of the axis.