Black Diamond Mines Regional Preserve encompasses an area that used to be the largest coal mining district in California (during the second half of the 19th century). The park contains 12 coal mines and the sites of what used to be 5 towns (okay, maybe "villages") that were supported by them. Today, the only things left to be seen are piles of mine tailings, some mine entrances, the flattened foundation sites of some buildings, a reasonably intact cemetery, and (reportedly) some railroad beds. It might seem surprising at first to see that not a single plank of wood of any building from the coal mining years remains today, at least until it is explained to you that it was common then for folks to disassemble their wooden buildings and take the materials with them when they moved away from a spot...

You can actually take a guided tour of a mine at Black Diamond Mines, albeit not of a coal mine but of a sand mine. Hazel-Atlas Mine was used for the extraction of sand suitable for glass production in the first half of the 20th century. Tickets for a one-and-a-half-hour guided tour of this mine can be purchased (for $3 per person, as of this writing) at the Sydney Flat Visitor Center in the park.

In terms of mountain biking, what the park offers is not too different from many other East Bay parks: Only fire roads are legal, the terrain is hilly, the scenery is idyllic when the grass is green and arid when its yellow, most climbs are steep, and views occasionally open up from hill tops to surrounding valleys and parts of the bay. One additional thing offered by Black Diamond Mines is the historic interest it holds. Those of you who like "bike-based exploration" might get the most kick out of this one.

This ride starts from the parking lot near the Somersville town site. (Basically, this is the farthest point you can reach by car when you enter the park via its main entrance.) There is a $5 entrance fee (as of this writing) that you need to pay as you enter the park this way. This is collected at a staffed toll booth. I'm not sure what the practice is in terms of paying the fee if you enter the park when the booth is unattended, or even if you can.

I believe that this particular route does a reasonable job avoiding steep climbs (with one notable exception). The first 0.75 miles of the ride climbs the "stem" of this, basically, lollipop-shaped route, on Stewartville Trail. The grade of this climb probably averages something around 9%, except for its last stretch that's more like 15%.

Ridge Trail greets you with a single very-steep initial burst (exceeding 20% grade), after which it starts following a ridge top mostly flatly or slowly descending. Along the way, you pass an intersection with Corcoran Mine Trail, which descends from the ridge to the valley floor more directly. This trail is probably the closest thing to a legal singletrack trail that I saw during this ride. I didn't take it, though. It might be worthwhile for me to try it some other time.

As you descend along the ridge top following the route shown on this page, views are available to the Delta and the Central Valley. Instead of descending all the way to the tip of the ridge, however, this route turns left onto Contra Loma Trail and takes a more roundabout way of reaching (what I'd like to call) "Stewartville Valley". The only point of this is to extend the ride a little bit and take it through slightly different terrain. Bypassing this part to descend to Stewartville Trail directly via Ridge Trail would save you about 1.2 miles of distance and 250 feet of elevation gain.

As you start heading west on Stewartville Trail, you'll initially find yourself alternating between riding flatly and imperceptibly descending on your way through the valley. You'll notice that there is a spot on the route plot where two spurs go off the main loop in opposing directions from the same spot along this part of the loop. The shorter one heading north is a (1500-foot) hike up to Prospect Tunnel. This tunnel is described as a 400-foot-long tunnel dug in the 1860s to look for coal. You can walk as far as 200 feet into this tunnel. It's the only mine tunnel in the park that can be freely entered by the public. If you'd like to try your hand at this (and you might want to see the photos of this ride in order to decide), you'll need to bring a flashlight with you; not to mention possibly a bike lock to hitch your bike to a nearby tree while you take the hike.

The longer spur on the other side takes you to the entrance of Star Mine. This is described on the park map as one of the last active coal mines in the area. That description might get your hopes up, but you still don't get to see anything here more than a narrow barred-up mine entrance through which you might be able to peer. (Again, check the ride photos for yourself.) This will cost you an extra 1 mile in riding distance and about another 250 feet of elevation gain.

Continuing along the loop, you'll soon be passing by the Stewartville town site. There's nothing to see there except an interpretive signboard and a pile of tailings from Central Mine.

Shortly after that, the route takes a sharp right turn and has you heading toward the most challenging climb of this ride. As the fire road climbs back to the intersection where you started the loop on Ridge Trail, it takes a few wide switchbacks where the grade averages around 13%, but, more importantly, frequently exceeds 20%. This whole section is no more than half a mile in length, though, so it's no big deal to simply walk it.

At this stage in the ride, all that's left for you is the descent back along that initial stretch of Stewartville Trail returning to the parking lot. You'll also notice a short spur in the GPS track from this intersection onto Carbondale Trail. That was only a short trip I took to look at what's on the northern side of that ridge, and certainly doesn't need to be emulated.

As long as you're willing to continue riding on fire roads, Black Diamond Mines offers plenty of choices for extending your ride. One option that seems to be fairly popular is a second loop by taking Nortonville Trail west and continuing on (and returning via) Black Diamond Trail. This will also allow you to see Rose Hill Cemetery (not to be missed, if you're at all interested in the historic aspect of the park) and "Jim's Place". I've been told that this is a comparatively deserted part of the park and the trail surfaces in this area are more pockmarked on average than the trail along the loop described above. I haven't ridden there myself to verify this firsthand. There are, however, miles of other loop options in the northern half of the park's trail network as well. Examine the map and improvize...