Direct evidence of the geologic age of the rocks constituting the type section of the Modelo formation in Ventura County, California, has never been recorded in the literature. The type section of this formation exposed between Sespe Creek and Piru Creek, along a line through Oat Mountain and the head of Modelo Canyon, consists of a series of sandstones, silty shales, siliceous shales, and cherts. This series of rocks has been described as resting conformably upon a formation containing a Vaqueros (lower Miocene) fauna, and as being overlain conformably by rocks characterized by fossils of Pico (lower Pliocene) age. As no unconformities have been found within the type section, it has been assumed by most of the geologists who have studied these rocks that the Modelo forma ion is a continuous series representing deposition during all of Miocene time subsequent to the Vaqueros period.

Fossils indicative of Temblor (middle Miocene) age have recently been found at two localities in the lower Modelo sandstone of the type section. A fauna indicative of uppermost Miocene (upper San Pablo) age has been found in a sandstone bed in the upper shale member of the type section of the Modelo. This sandstone rests with seeming conformity upon the underlying beds. At other localities, north of the type section, rocks formerly believed to be of Pico age, but which are now known to carry fossils indicative of uppermost Miocene age, rest with marked unconformity upon the underlying Modelo strata. The assumption of previous workers that the type section of the Modelo formation represents continuous deposition from post-Vaqueros time to the close of the Miocene is, therefore, amply s pported by structural and paleontological evidence. At other localities a diastrophic break is evident at the base of the uppermost Miocene.

Our conclusion is that the Modelo formation of the type section consists of three divisions, which have the rank of formations. The lower division is of Temblor (Topanga, middle Miocene) age; the upper division is of uppermost Miocene (upper San Pablo, uppermost Miocene) age, whereas the middle division, which has not yet yielded fossils, is believed to be of lower San Pablo (Briones, lower part of upper Miocene) age.