Monterey Formation

The Monterey Formation (colored yellow here) is widely distributed over the Pt. Reyes Peninsula. Evidence has been obtained that although superficially similar to the Monterey, the rocks of the Bolinas area mapped by Galloway as Monterey are actually much younger.

 The Monterey Formation of Pt. Reyes is strongly correlated with the type locality deposits in the city of Monterey, California. Thus the suggestion that Pt. Reyes was still in the Monterey area up until Miocene times.

Galloway's Monterey Formation (Tm) Description:

Siliceous shale and minor chert; shale weathers white to gray or pinkish brown, and is commonly laminated to thin bedded, variously micaceous, silty, porcelaneous, or cherty, and locally contains calcareous concretions or sandstone or siltstone interbeds; chert is dark gray-brown and laminated with thin shale and sandstone interbeds; fish scales, carbonaceous material and molds of foraminifers, diatoms, and fish remains are variably present.
Clark and Blake description:
Monterey Formation -- Thin-bedded and laminated, light-colored porcelanite with very thinly laminated shale interbeds and few thin to medium interbeds of biotitic arkosic sandstone is as thick as 450 m on the southwest slope of Inverness Ride.  Interbedded light-gray porcelanite, brown brittle chert, and dark-yellowish-brown mudstone with scattered dolomite concretions that form the upper part of the Monterey are estimated to be from 450 m to 900 m thick (Clark and others, 1984).  The Lockhart well on Bolinas Mesa (well no. 6, Sheet 1) penetrated as much as 1500 m of the Monterey.  This formation yields benthic foraminfiers diagnostic of bathyal or greater depths and of Luisian and Mohnian (middle and late Miocene) age with most of the Monterey being Mohnian (Clark and others, 1984).

Up section to the angular unconformity