The structure of the oil-producing Pliocene strata of the developed McKittrick field, together with several square miles of related underlying Monterey Miocene shales on its western border, are concealed by an over-riding mass of disorganized Monterey shale from the middle adjacent slopes of the Temblor Range. The interpretation of the structure of the McKittrick field as given by Ralph Arnold and Harry R. Johnson, in United States Geological Survey Bulletin 406 (1909), is that of thrust faulting with movement of Miocene strata over Pliocene in an easterly direction.

The writer summarizes briefly the geology of the McKittrick area and presents his conception of the structure as here outlined. The formations, in order of age, and their thicknesses are: (1) Basement complex, schist, marble, and granite, (2) lower Miocene, Temblor sand and shale, 1,200+ feet, (3) middle and upper Miocene, Monterey and Santa Margarita organic shale, 5,000-7,000 feet, (4) lower Pliocene, Etchegoin sand and shale, very thin to 1,000+ feet, (5) upper Pliocene, Tulare sand and shale, very thin to 1,000+ feet.

Beginning in the Pleistocene, a very large segment, approximately 1 mile wide and 6 miles long, of the eastern front of the Temblor Range, composed here of a great thickness of crumpled and fractured Monterey (Maricopa) shale, was moved by gravity northeastward and downward, nearly 2,000 feet, for a distance of 2-3 miles over the eroded edges of the upper Miocene and Pliocene strata, and overlapping and capping the lower Pliocene oil-bearing beds which now produce the oil of the McKittrick field. The gravity movements of the Monterey shale down the slopes of the mountains have been progressive and are now going on periodically in the upper part of the broken mass of shale at elevations ranging from 1,500 to 2,000 feet above the McKittrick field