East of St. Helena near Angwin on Howell Mountain, theres a challenging network of trails through dense forest of manzanita, madrona, redwood and oak. This combination of rolling singletrack and fire roads is probably the best riding around Napa Valley.
To reach the mountain biking trailhead from St. Helena, follow Deer Park Road east across Silverado Trail for 5.3 miles. Deer Park Road eventually merges with Howell Mountain Road. Near Agwin, turn right on Cold Springs Road and then veer left onto Las Posadas Road. After 1 mile, park near the sharp right turn at the end of the vineyard.
This unmapped trail network can be confusing so remember that youre starting near the top -- so up is usually out. Watch out for the great bonzai downhill runs dropping into distant valleys.
Overall, Las Posades / Pacific Union College Trails is the 25th most popular mountain bike trail of all 132 mountain biking rides in California.