The upper member of the typical Monterey Formation near Del Monte and Monterey, California, bears a distinctive diatom flora of at least 288 species. Most of these diatoms are bottom-dwelling forms; only a few are sessile. This flora is of Delmontian (late Miocene) age.

In the "Sisquoc" Formation along Harris Grade near Lompoc, Santa Maria District, three diatom floras may be distinguished: all are younger than the Delmontian flora of the upper typical Monterey, but only about 150 species occur. Pelagic forms are most numerous in the older and middle of these "Sisquoc" floras whereas bottom-dwellers, both free and attached, increase upward through the sequence, being most common in the youngest of the floras.

All four of these distinctive diatom floras have been found over an extensive area in the California Coast Ranges. They occur from Monterey in the north to Taft in south-central California and to Santa Barbara in the southwestern part of the state.

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