The South Fork Mountain Schist at Black Butte and Cottonwood Creek consists largely of fine-grained metasedimentary and voluminous mafic metavolcanic rocks. Contacts between the metasedimentary and metavolcanic rocks are commonly thrust faults that formed during or preceding blueschist-facies metamorphism.
The contact between the South Fork Mountain Schist and “normal” Franciscan metagraywacke was mapped by Suppe (1973) in the Ball Mountain region as the Log Spring fault. This fault may be traced northward into the Yolla Bolly quadrangle where it diverges westward from the schist-graywacke contact. At Cottonwood Creek the schist apparently passes down gradationally into less-metamorphosed rocks.
The primary rock types of the South Fork Mountain Schist have more similarities to the lower facies (Knoxville Formation) of the Great Valley sequence than to other units of the Franciscan Complex, in that its primary sedimentary rocks were mainly mudstone, rather than graywacke. The schist may represent a similar facies relationship to the Franciscan as the Knoxville does to the Great Valley, namely, an older distal portion; or it may have originated as part of the Knoxville. In either case, the nature and position of the schist are more compatible with a western rather than an eastern source for the Franciscan graywackes.