The first drilling for oil in the Elk Hills commenced in 1910. The first well to produce oil was completed in June, 1911, at a depth of 4,000 feet. At the end of June, 1928, the total production was 96,199,914 barrels. The surface beds, aside from alluvium, belong to the Tulare formation of uppermost Pliocene age and are divided into upper, middle, and lower zones, altogether 730 feet of fresh-water sand, marl, and clay. The subsurface beds are thicker and, in addition to the lower Pliocene, include Miocene formations. Maricopa diatomaceous shale (Miocene) is supposed to be the source of the oil. Surface structure, reflected by topography, is an elongate dome with closure of 450 feet. Subsurface structure much steeper than the surface indicates a closed dome in the easter field, in the western field, and in the Hillcrest area. Anticlinal conditions have influenced the accumulation of the oil and gas; but sand conditions, more than position on structure, have influenced the volume of production.