Topanga Canyon is part of the Topanga Canyon Formation. It contains middle Miocene sandstone and siltstone.
The sediments contain large quantities of gastropods and pelecypods (most are shell prints). It is recommended to go after a rain when the fossils have been washed (at least partially) out of the matrix.
“from the Ventura Freeway (in the San Fernando Valley), get off at the Valley Circle/Mulholland Drive exit Turn left onto Valley Circle, cross over the freeway and proceed onto Mulholland Drive. Turn right onto Valmar Avenue — Valmar becomes Old Topanga Canyon Road. At Mulholland Highway, bear right and prepare to turn left back onto Old Topanga Canyon Road in 1/8 mile. Turn left and proceed up Old Topanga almost a mile, past the point where the road switches back upon itself. From this point until the top of the hill, you will encounter fossiliferous road cuts on your right. The first embankment contains a massive oyster reef.” (see first citation for source)
Sam Park, “Fossil Collecting in California” (The Fossil Forum 2008) http://www.thefossilforum.com
“Topanga Canyon fossils,” (Rockhound Blog October 12, 2006) http://www.rockhoundblog.com