The Reef Ridge shale is restricted in this paper to a distinct lithologic unit essentially the same as the "caving blue shale" of Kettleman Hills and as such deserves recognition as a formation. From references made it is apparent that this was the original intention although the type locality, as described, included the upper member of Henny's previously described McLure (brown) shale formation. Field studies indicate that the formation as thus restricted is for the most part only about 300 feet in thickness and the upper and lower contacts are both gradational considering the definitions of the adjacent formations. Foraminiferal studies by R. M. Kleinpell, reported in this paper, have checked the original Miocene classification of Barbat and Johnson for at least the low r half of the formation at the type locality. The age of the upper part is not definitely known but it is presumed in general to be transitional between the Pliocene and Miocene as these terms are used in California at the present time following the general classification of B. L. Clark. The recognition of the formation in the other parts of the old San Joaquin Valley basin is considerably complicated by facies changes in the overlying beds. Because of the lack of suitable marker beds and fossils, in the Central Valley region, the writer suggests the arbitrary line at the top of the vaguely fissile silty shale as the upper contact of the formation in that area.