The following description was written by Michael DeMicco (demicco@value.net):
I live within a few miles of Black Diamond Mines park in Antioch, and have ridden here in the hundreds of times. Black Diamond was once a thriving coal and sand mining area. Now, all that remains are an old cemetary and the remnants of the old mines. The area is nearly 3700 acres of grassy, oak studded hills and some rock formations. It has some excellent local riding. It can get pretty hot here in the summer (go early and take two large water bottles and some food with you). It's also known for its death mud (adobe clay). In the winter, after the rains have hit and the cattle have hoofed-out the trails, it's pretty much unrideable. (I once saw this guy ride down the "Wall" and his bike came to a complete stop in a hoofed-out area of death mud. He got off his bike and the bike remained upright.) The trails are graded around the beginning of June. Normally its a great place to ride. You can get rides exceeding twenty miles in length with tons of climbing and lots of high speed descending. The following is what I call the double loop ride. It's about fifteen miles long. Either of the two loops can be ridden individually, or you can bailout midway if you decide the ride is too much of a good thing. As always, control your speeds on the descents and watch out for hikers and horseback riders.
Start at the parking lot on the left past the entrance kiosk at the Sommersville Rd. entrance in Antioch. Go up the Railroad Bed Trail (which parallels the paved road) till you reach the upper parking lot. The Railroad Bed was (is) just that, and the gradual climb will get you warmed up before the serious climbing begins. Once you get to the upper parking lot you might want to get a park map. Check out the entrance to the old Hazel-Atlas mine while you're there. You can get water midway through the ride here as well. Continue on the Nortonville Trail and climb past the old, abandoned Rose Hill Cemetary (stop to check it out, if you wish). After the cemetary climb, go over the top (bear right, don't take the Black Diamond Tr. to the left; you'll be returning that way), continuing on Nortonville Trail on a fast descent. At the bottom, go through the gate to the left (Black Diamond Trail). Take this trail until it intersects with a paved road. Continue climbing on a steep paved road (called the "microwave towers" climb) to the left until it ends at a locked gate. Take the dirt trail to the left (still Black Diamond Trail at this point). Now be prepared for a lot of down (with some up). At the trail juncture (you will see the cemetary ahead), take the Nortonville Trail down to the upper parking lot area at the end of Sommersville. Here you have finished the first loop. Head down Sommersville or the Railroad Bed Tr. to your car, or start the second loop.
From the upper lot, retrace your path up the paved trail, and make your first left onto a dirt fire road called the Stewartville Trail. Continue through the gate and climb straight up to a saddle where there is a junction with other trails. Go through the gate and make a left turn onto the Ridge Trail (you'll be returning up trail to the right). This is the best way to do this loop, because descending the Ridge Trail is a blast. After a short, steep climb, prepare for a roller coaster ride of epic proportions. After the ride is over (descending, that is), make a right onto the Stewartville Tr. at the bottom of the Ridge Trail (to the left goes to Contra Loma Resevoir. (You may want to go down there for some added miles and also to get water.) Continue a gradual ascent on Stewartville Tr. through the Stewartville Valley until you reach the "Wall". Be prepared for some serious climbing. The "Wall" is aptly named and has short, flat relief section half way up so you can recover. After grinding up the second section, you reach the multi-trail intersection you were at earlier. Go through gate and head straight down. Beware of hikers on this section and watch your speed. End at the upper parking lot and either take the Railroad Bed Tr. or Sommersville Rd. back to your car.
"It is off of Marsh Creek Road out in Clayton. It is on the left if you are heading toward Byron just before where they are building the new Oakhurst Clubhouse. The trail goes from there to Kirker Pass Road."
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