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. Continue straight across the road for another 0.1 to a fork in the trail. Stay right at the fork and continue about 0.4 mile until you see Mossy Rock on the left.