Sedimentologic study of the Simmler Formation and the transition to the conformably overlying Vaqueros Formation was undertaken in order to reconstruct the Oligocene history and paleogeography of this part of the southern Coast Ranges of California. The Simmler, of Oligocene and possible latest Eocene age, consists of a sandstone facies in the Caliente Range, and a conglomerate facies to the west, southwest, and southeast around the margins of the basin. The sandstone facies is composed of greenish-gray sandstone and red and green claystone or mudstone in fining-upward cycles that average 2.5 m thick. The conglomerate facies consists of reddish muddy conglomerate to pebbly mudstone and interbedded sandstone. Transgressive marine strata overlying the Simmler, and laterally equivalent to its uppermost part, are included in the Vaqueros Formation of late Oligocene to early Miocene age. The lowest Vaqueros sediments are nearshore or strandline sandstones that are overlain, in the Caliente Range, by deep basinal siltstones. The sandstone facies of the Simmler is interpreted as an alluvial plain depositional complex formed by through-flowing streams; the conglomerate facies represents alluvial fan deposits rimming the alluvial plain. The streams were probably of low sinuosity and of small to moderate size. Petrographic and paleocurrent data indicate that the Simmler Formation and the overlying Vaqueros Formation were derived from the same source terranes and that the sediments were transported in the same general direction. The conglomerate facies of the Simmler provides evidence of late Oligocene activity on the Nacimiento, Ozena, and Blue Rock faults; the Chimineas-Russell fault was apparently not active at this time. Although this study provides no direct evidence bearing on offsets along the San Andreas fault, sedimentologic evidence from the Simmler tends to support the hypothesis that the San Andreas fault did not come into existence in this area until after the Oligocene. The most significant tectonic event during the late Oligocene in this area was the inception of basin formation evidenced by the foundering of the Caliente Range area in the late Zemorrian that apparently resulted from interaction of the Pacific and North American plates.