Ultimate Guide to Finding Fossils at Mt. Diablo

Geology of the Area:

According to the Mt. Diablo Interpretative Association, Mt. Diablo is a mountain with an elevation of 3849 feet in the Diablo range. Although the mountain has only very recently emerged, the rocks it is composed of are very old. Due to its complex geological history, the rocks of Mt. Diablo can be divided into three groups, Mt. Diablo Ophiolite (Jurassic), Franciscan Complex (Jurassic and Cretaceous), and Great Valley Group (Jurassic and Cretaceous). Additionally, the younger sedimentary rocks on the higher strata are Cenozoic.

Mt. Diablo Ophiolite: An ophiolite is a part of oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often developed onto continental crustal rocks. According to radioactive and fossil-age determinations, the ophiolite emerged approximately 165 million years ago. The ophiolite basalt has a crystalline feel with a black to greenish-brown hue. This is created when lava erupts underwater and ‘freezes’ when it makes contact with the water. Diabase is a coarser version of the ophiolite basalt that is created by lava erupting from vertical fissures underwater. Serpentinite can also be found on Mt. Diablo.

The middle area of the Mt. Diablo strata known as the Franciscan Complex is the result of 140 million years of east-dipping subduction. Concerning fossils, approximately 10% of the Franciscan Complex is composed of shale. Unfortunately, most of it has been converted to argillite as a result of metamorphism.

The Great Valley is composed of sedimentary shale and sandstone from the Upper Jurassic to the Cretaceous period. It is mostly deep-water shale. There are also younger Cenozoic strata on top. As a result of the drying of Central Californian basins during the Miocene and Pliocene, marine fossils end at that time.

Paleontology and Fossils:

Cow and Aelurodon jaws have been found, in addition to plants. Marine fossils from both deep and shallow water environments have been recovered.

Directions to Finding Fossils at Mt. Diablo:

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.” (Geohiking.com)

Once you arrive:

“From the parking area, you will notice the equestrian and cyclist trailhead across the road. The hikers’ route is past the gate on the paved road into Castle Rock Park. You can take either. 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 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.

A short distance further you arrive at a rock on the right with a hole bored through it by water erosion. You won’t be the first person to climb up into the round room inside the rock.

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.

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.” (Geohiking.com)

Other Fossil Sites Nearby:

Other fossil hunting sites nearby include Bolinas Lagoon, Pleasanton Sunol Road, and Half Moon Bay.

Disclaimer: California Fossils focuses on hosting a user-friendly interface and a flexible platform where your finds can be published almost instantly. However, we realize that increased traffic to fossil sites leads to erosion and degradation of sites, which is why we strongly recommend not removing fossils and leaving them where they have been found for others.

Works Cited:

“Castle Rock”, Geohiking.com, http://geohiking.com/castlerock.html

“The Rocks of Mt. Diablo- Their Type and History,” (Mt. Diablo Interpretative Association 2016) www.mdia.org