Castle Rock

EFFORT Minimal
LENGTH3.0 miles
GEOLOGICAL FEATURE(S)Erosional Features, Fossils
LOCATIONDiablo Foothills Regional Park
TRAILHEAD COORDINATES 37.53.541N 131.59.579W

Driving Directions.

Description: This nearly level, wide road is popular with equestrians and bicyclists, as well as local residents out for a weekend stroll. For us, it is a visit to stunning sandstone outcrops and marine fossils.

From the parking area, you will notice the equestrian and cyclist trailhead across the road. The hikersí route is past the gate on the paved road into Castle Rock Park. You can take either. The trail above skirts around the main park area and also allows you to avoid climbing a gate, but the gate is easy, so donít worry about it. Walk up the paved road into the park past the picnic area, swimming pool, and park buildings. This is a private park, but hikers are allowed trail access through it. The road turns to gravel past the restrooms. You will be walking through Pine Canyon alongside Pine Creek all the way with moderate shade. The landscape is primarily rolling grass-covered foothills of Mount Diablo with oak trees providing contrast. The trail is an old wagon road called Stage Road (and signed that way as well).

Leaving Castle Rock Park behind, you will pass a gate and continue to a fork. Bear right to stay on Stage Road. Before long you arrive at a sandstone wall on the right which clearly exhibits bedding. The rock wall is tilted to almost vertical and several distinct layers are revealed.

A short distance further you arrive at a rock on the right with a hole bored through it by water erosion. You wonít be the first person to climb up into the round room inside the rock. Itís a cozy little den with two windows, big enough to sit in. The creek is on the left, and beyond that is the beginning of the intriguing series of sandstone crags known as Castle Rock. Notice the many caves and holes in the cliff face. If you have your binoculars, you will be able to see that the holes are used by nesting birds. The white streaks beneath the holes are a giveaway. Hawks, including redshoulders and Coopers, are often seen in the oak trees near the trail. You may also see golden eagles.

There are many creek crossings on this trail, seven of them, and it may not be much fun from December to March, depending on rainfall and horse traffic. In fall, the first several of them will be dry, but during rainy season, you will get wet fording them. There are also a couple of gates which you may have to climb over when locked.

After the first stream crossing, you will spot a narrow footpath leading to the right through the grass. In spring when the grass is growing, this path may not be apparent. If you come to the second creek crossing, you missed it, but by turning right and following the edge of the creek a short distance, you will be there. Take the footpath past a grape arbor of sorts to a slide in a tall cliff above the creekbed. There is a rich layer of marine fossils in this cliff. Some of these wash down to the creekbed each year. If you look carefully for whitish rocks, you will find pieces of this exposure. Thick clusterings of mollusk shell fragments can be seen. This sort of rock, composed mainly of shells with some sandstone is known as a "hash" for somewhat obvious reasons. Shell hashes in the Diablo foothills are generally of the Briones Formation. You can see these on Rocky Ridge in Las Trampas Regional Park, Fossil Ridge on Mt. Diablo, and various other locations in the area. If you look closely at the shell pieces, you may find some that are large enough to recognize as a clam, the dominant type of fossil. Pectens, the bivalve mollusk made famous as the symbol for Shell Oil Company, also occur, but are less common.

Why are the shells so completely shattered? Deposited in a shallow marine environment, the shells were slammed about by waves, turbulent storm currents, smashed on rocks, and generally abused until finally coming to rest and being buried.

Fossil Hash

It was at this detour to the fossil site that we came upon a bobcat, and got within twenty-five feet of it before it sauntered off along the creekbed. It was also very near here that we saw a clutch of about twenty quail in the underbrush. Mountain lions have reportedly been spotted in Pine Canyon over the years, but it will be quite unusual if you see one.

Returning to the main trail, continue to the second creek crossing. Good views on the left of Castle Rock are still with us as we ford the stream for the third and then fourth time, keeping Pine Creek and the Mount Diablo State Park boundary to the left. After the fourth crossing, a couple of picnic tables are available for a rest or snack.

Streams following the fourth crossing contained a small amount of running water, but it was easy to step across on rocks. Eventually you will come to Pine Pond, a reed-choked body of water frequented by ducks. This is the turnaround point.

On the way back, after passing the picnic tables, you may want to descend to the creek below to the right. There are several footpaths down to the trail there which edges right up to the state park boundary fence. This detour will keep you away from speeding bicycles and horses.

Driving Directions:In Walnut Creek on Interstate 680, take Ygnacio Valley Road east to Oak Grove Road. Turn right (south) and continue as this road becomes Castle Rock Road. Proceed to the end of the road where it is blocked by a gate and there is parking on the left. Access is free.

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