Angwin has two small, developed climbing sites, plus a few scattered boulder problems not included here. The rock is pocketed volcanic tuff similar to that of nearby Mt. St. Helena.
Mossy Rock is a boulder up to 20 feet high located in mixed evergreen forest, and offers bouldering and top-roping ranging from 5.7 to V4 (5.12). The approach (0.6 mile) is on a good, flat trail.
Bell Canyon has a west-facing band of cliffs offering lead climbing up to 45 feet high, with both trad and sport climbs ranging from 5.6 to 5.11. The approach (0.5 mile) is difficult with sections that are steep, loose and bushy, and the climbing is mediocre.
The climbing history is obscure. The easier climbs were probably pioneered by faculty and students of nearby Pacific Union College. The harder climbs were established by the early 1980s by Tony Yaniro (climbed first 5.13 in North America), Mark Spencer (author of "Southern Yosemite Rock Climbs") and the late Jim Hanson (rock climbing instructor at PUC), among others.
MOSSY ROCK: Boulder located on property owned by Pacific Union College, currently accessible to the public. From the junction of Silverado Trail and Deer Park Road in St. Helena, drive 5.4 miles north on Deer Park Road, which eventually becomes Howell Mountain Road, to the campus of Pacific Union College. Turn right on Cold Springs Road and, after 0.2 mile, continue straight across an intersection onto Las Posadas Road (Cold Springs Road turns right) for another 0.9 mile to a 90-degree bend in the road. Park at the bend of the road and hike on the trail that proceeds east (the same direction you were just driving). After about 0.1 mile you cross a fence and a dirt road, then reach a fork after another 0.1 mile. Stay right at the fork and continue about 0.4 mile until you see Mossy Rock on the left.
BELL CANYON: Band of cliffs located on property owned by St. Helena Hospital, currently accessible to the public. From the junction of Silverado Trail and Deer Park Road in St. Helena, drive 4.1 miles north on Deer Park Road, which eventually becomes Howell Mountain Road, to White Cottage Road. Turn left on White Cottage Road and continue 1.6 miles to the junction of Liparita Avenue on the left and Brookside Avenue on the right. Turn left on Liparita Avenue and drive 0.3 mile until you see some parking spaces on the left just in front of a gate and beside a pump house. Walk back toward White Cottage Road for about 20 feet to the end of the fence on the right side of the road and pick up the trail behind the fence. Follow the trail through some manzanita past a pump house, then downhill on a wide, zig-zagging trail about 0.3 mile. Shortly after crossing a stream, look for a pipe that exits a steep bank on the right side of the trail. Exit the trail at this point, boulder-hop across the stream through blackberry vines, and follow a climber's trail 0.1 mile across chaparral and down a couple of steep boulders to the base of a cliff. Scramble 0.1 mile down, then up along the base of the cliff, cross a steep sandy slope (potentially unstable and dangerous after a heavy rain), and continue along the base of the cliff to the main face.