These castles of tilted seabed on the far side of the Golden Gate are, gloriously, only a 30-minute morning bike ride away. But truly communing with the chevron chert means getting off the bike, locking it, and going for a run on the Headlands' seaside trails. Here's what you do: Ride across the west-side bike lane on the Golden Gate Bridge, go through the gravelly parking lot that follows the bike lane, then turn left at Conzelman Road. For the next mile and a half you'll struggle up a 12 percent grade until you reach an overlook parking lot, and the start of a single-lane, cliffside descent that has to be one of the most spectacular in the Bay Area. At the bottom, turn right onto Bunker Road and lock your bike in the dirt parking area just off the Rodeo Valley Trail. Stash your bike shoes in the bushes and lace up your running shoes. Then light out along the Rodeo Valley Trail eastward and upward 2.2 miles until you encounter the Alta Trail, which runs along the ridge above Sausalito. After less than a mile you'll merge onto Bobcat Trail; another mile or so onto Marincello Trail, which takes you on a two-mile descent into Tennessee Valley. You'll see families picnicking and hikers hobnobbing. But you'll keep running, passing through the horse stables up onto Old Springs Trail, a fairly steep climb that two miles later meets up with the Wolf Ridge Trail. Turn right, run another mile or so upward toward the old abandoned Nike missile installations, until you hit the Coastal Trail. Run a mile and a half downhill to Rodeo Beach, then another mile or so along Miwok Trail, and you'll be back to your bicycle. You'll have climbed 2,200 feet on foot, with only around 45 minutes of bike riding to go: Ride your bike up the Coastal Trail to Conzelman, head back across the bridge. Believe us; you'll have had enough chert-communing for one day.