This hike is a loop that ascends to the top of Shell Ridge, travels along the top of the ridge, descends into Pine Canyon, and follows Pine Creek back to the trail head. You will most likely see hundreds of California ground squirrels and the predators that rely upon them.
Begin at the Orchard Staging Area which is the grassy parking area roughly 1/4 mile from the end of Castle Rock Road. Head due south through the cattle gate, and a large ranch will be to your left across the road, and a grassy hill looms on your right. I've seen coyotes a number of times on this hill. You quickly reach a junction: head left to continue on the Castle Rock Trail, or go right to reach the Shell Ridge Loop Trail. After turning right you begin a steady ascent to the top of Shell Ridge. 1/4 mile from the junction you reach another junction. Turning right takes you north on the Borges Ranch Trail and back to Castle Rock Road, but you want to turn left to remain on Shell Ridge Loop.
At this point you've basically reached the top of the ridge, and there are expansive views in all directions. Briones Regional Park is to the west beyond the city of Walnut Creek, and the view east of Mount Diablo is possibly the best view of that massif. Continue south along Shell Ridge Loop Trail while ignoring the spur trails that branch off east towards the bottom of Pine Canyon. After approximate 1.25 miles the Shell Ridge Loop Trail button-hooks to the east then north before reaching the bottom of a canyon that's a tributary of Pine Canyon. There's a 3-way junction: go left to reach the bottom of Pine Canyon or right to ascend to the Briones-Mount Diablo Regional Trail. After turning right you reach another 3-way junction after a few hundred yards. Both routes take you to the Briones-Mount Diablo Regional Trail which is only 0.25 miles ahead.
Until this point you have been traveling through a mixture of mixed oak savannahs and deep oak forests. When you reach the Briones-Mount Diablo Regional Trail you enter a world of wide-open grasslands. This is the kingdom of the California ground squirrel. These are very large squirrels that live in burrows, and you are likely to see hundreds of them before you re-enter the forest 2 miles to the southeast. These beasties form the basis of a large biotic food web in the area. Coyotes are all over the place, rattlesnakes lurk in the dense brush, and it's impossible to not notice the myriad raptors soaring overhead. You may think that these squirrels are merely helpless prey for this suite of predators, and you'd be wrong. They stand on their hind legs in front of their burrows keeping both eyes trained on any coyote in the area. They kick sand in snakes' faces, rub themselves on snake skin to confuse the snake's sense of smell, and even directly confront these predators with their large bushy tails above their bodies to make themselves appear too large to eat. Their eyes are constantly trained skyward on the lookout for red tail hawks. Alas, many many ground squirrels fall prey to these predators, and the numerous turkey vultures are there to clean up after a kill.
The Briones-Mount Diablo Regional Trail continues along the spine of Shell Ridge to the southeast through these grasslands. I'm not kidding when I claim that you're likely to see hundreds of ground squirrels, and you'll undoubtedly also see a few cows. This is not a hike for warm weather due to the fact that there is almost no shade. You eventually reach a junction with the Wall Point Road (actually a fire road that is typically only used by hikers and mountain bikers). Above this junction is the China Wall which is a collection of sandstone monoliths that jut out of the underlying grassland like a dragon's teeth.
The Wall Point Road continues almost level before you turn left onto Dusty Road (another fire road). The Wall Point Road continues to the southeast if you're interested in a longer loop. Dusty Road travels through a dense forest of oak and Coulter pines. The Coulter pine has the largest cone of any pine species, and you would be in serious trouble if one hit you on the head. They commonly weigh 5 pounds, but the good news is that Coulter pines don't grow very tall so a falling cone wouldn't be moving too fast by the time it bonked you on the noggin.
Dusty Road reaches the bottom of Pine Canyon about 0.5 mile after branching off from Wall Point Road: go right if you want to head up to the high country of Mount Diablo, but turn left to return to the trailhead. The trail along the bottom of Pine Canyon is Stage Road (fire road), and it follows Pine Creek. You'll cross that creek at least 6 times, and some of them are fairly scary when the water is flowing well in the spring. You travel roughly 2 miles northwest on Stage Road before reaching a junction with the Shell Ridge Loop Trail. A few hundred yards past this junction branch off of Stage Road go veer to the left at a fork. Going right takes you down to the bottom of Pine Creek and the ball fields, swimming pool, and picnic areas at Castle Rock Regional Recreation Area. Veering left takes you through a dense forest of oak before reaching the cattle gate at the south end of the Orchard Staging Area. - Apr 09 2011 Jim Lyding