From CA 4 in Contra Costa County exit Somersville Road (exit 26a eastbound/26 westbound). Drive south on Somersville to the park entrance.
If you’re driving from San Francisco, or the south bay, these directions may make your trip quicker (or at least more pleasant): from the CA 24/Interstate 680 interchange in Contra Costa County, exit Ygnacio Valley Road (exit 46b). Drive east on Ygnacio Valley Road about 8 miles, then the road changes into Kirker Pass. Continue about 5 miles on Kirker Pass to the junction with Buchanan. Turn right and drive about 3 miles to Somersville Road. Turn right and drive to the park entrance.