The Miocene to Pliocene Angular Unconformity

The Monterey Formation (and all older rocks) suffered deformational activity that folded them, likely into subaerial exposures where erosion took its toll. Following this erosion, the land was submerged and new marine deposits (the basal green sands of the Drakes Bay Fm) were deposited upon the tilted and eroded Monterey Fm.

A good locale to seen this is on the south end of the Limantour Beach where, at low tide, the green sand unit can be seen blanketing the upturned and eroded Monterey Fm.

Another location where, with some imagination, you can "see" this angular unconformity is where the main road west to the Lighthouse crosses Drakes Estero. Upturned Monterey beds are visible on the margins of the tidal channels while in the background the horizontal Drakes Bay Formation can be made out in the adjacent hills.

These same basal green sands can be seen lying non-conformably on granites of the headlands near Chimney Rock. We do not know if the Monterey was completely eroded away from this location or that it was never deposited here.


Up section to the Drakes Bay Sandstones