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Mt. Diablo State Park Exploration

This park is one of the ecological treasures of the San Francisco Bay Area.
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Difficulty: Strenuous
Length: 3.9 miles
Duration: 1-3 hours

Overview :  Many visitors to Mount Diablo head straight for the summit to enjoy the famous view. Summer days are sometimes hazy, and the best... more »

Tips: 

Location-Directions


The park may be accessed by vehicle from either the Danville area (Mount Diablo Scenic Boulevard) or the Walnut... more »

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Points of Interest

1. Radiolarian chert folds, or "Chevrons"

Mt. Diablo is famous both for its rich geological history and the many fossil species discovered there. Here you see beds of chert, which itself is made of fossils, the recrystallized silica skeletons of microscopic sea creatures called radiolarians. When the radiolarians die, their skeletons slowly settle to the sea floor, where they form an ooze... More

2. Nice example of blueschist

This blueschist is an unusual type of metamorphic rock that forms in high-pressure, low-temperature environments, such as plate subduction zones, where one of the earth's tectonic plates slides under another.

Blueschist is usually derived from basalt or graywacke sandstone that has been dragged down 20-30 miles, squeezed, and then faulted back up... More

3. Rest your head here? Pillow basalt

These pillow basalts formed where two plates of the earth's crust were under water and moving away from each other. Their blobby shape is characteristic of an underwater volcanic eruption, in which the outside of the flow cools and solidifies while the inside is still molten. The presence of volcanic rock on Mount Diablo can be confusing, since... More

4. Mount Diablo Manzanita (Arctostaphylos auriculata)

Botanically, Mount Diablo is an "island mountain" isolated on a relatively flat plain. The mountain is home to many endemic plant and animal species, including the Mount Diablo manzanita (Arctostaphylos auriculata), found nowhere else in the world. Some botanists think the peeling red bark of the manzanita is the tree's way of shedding... More

5. Mt. Diablo thrust fault

To a trained eye, this hump in the trail reveals tectonic secrets. Although not thought to be active, this fault known as the "Mt. Diablo thrust fault," is characterized by a break in slope, a change in rock type when going from one side to the other, and ground-up rock nearby. The fault is hard to see because it's been reshaped; the... More

6. Leap through time

Look both ways before you cross this street...you're about to step 25 million years ahead. You're standing on rock once dominated by dinosaurs (during the Cretaceous, 75 million years ago), and are about to cross to rocks that formed when mammals and flowering plants took over (the early Cenozoic, 50 million years ago), Geologists call a gap like ... More

7. Variegated Meadowhawk

Mount Diablo abounds with insect and arachnid life. One stunning example is the dragonfly Sympetrum corruptum, or the variegated meadowhawk. Dragonflies are ancient creatures. They existed even before the oldest rocks on Mount Diablo formed.

An open-eyed hiker can spot lots of well-camouflaged insects, like this European mantis (Mantis religiosa). ... More

8. Blue Oak (Quercus douglasii)

The deciduous blue oak (Quercus douglasii) is found only in California. It was first identified as a species by the famous explorer and botanist David Douglas. We know that Douglas visited Mount Diablo in the 1830s because he identified the Mount Diablo fairy lantern, a beautiful flower that grows nowhere else in the world.

Canyon live oaks ... More

9. Gall of Red Cone Gall Wasp

For the keen observer, a myriad of tiny jewel-colored formations can be found on almost every species of oak on Mount Diablo. These unlikely structures are called wasp galls, and serve as shelter and a food source for the larvae of a tiny wasp no bigger than a fruit fly. This example is from the red cone gall wasp (Andricus kingi). These are also ... More

10. Fossil Turritella Snail bed

Here you can see a three-foot-thick sedimentary rock layer containing the fossil snail, Turritella aedificata. During the Eocene period, 45 million years ago, these marine snails likely crawled along the ocean bottom and ate algae. This fossil deposit is an important marker for the geographic layers in Mount Diablo. Outcrops are seen a couple of... More

11. Sentinel Rock, Mt. Diablo State Park

These tan-colored sandy rock formations were formed in the Eocene, and wrap around the south and west sides of the mountain. Their arrangement indicates that they were laid down here about 37-58 million years ago.

The view from Sentinel Rock, Mt. Diablo State Park. Relatively undisturbed, this landscape has looked like this for thousands of years, ... More

12. 'Water' caves, not wind caves

Mt. Diablo State Park is famous for its stunning sandstone formations, also known as 'tafoni." Although commonly referred to as "wind caves," the element most responsible for creating these formations is water. Mt Diablo is probably only 2 million years old at the most, and the caves are only a few hundred or thousand years old.... More

13. Fossil scallop on the grill

An ancient fossil scallop shell peeks out from a surprising place, the barbecue pit at the Rock City campground. Fifteen million years ago, this area was on the eastern edge of a large inlet called the San Pablo Embayment. It was larger and deeper than San Francisco Bay, and was home to the ancestors of organisms we find in the bay today.

This 15... More

14. Gray pine cone up close (Pinus sabiniana)

The cone of the Gray pine (Pinus sabiniana) or Digger pine is one of the heaviest of any conifer, up to several pounds for a single cone, which could pack a whallop falling from this tall tree. Needless to say, you won't find a picnic table under a Gray pine anywhere in the park.



The Gray pine exhibits several characteristics of a drought-adapted... More

15. 15 million years ago - storm evidence

These bands of pebbles in the massive, fine-grained Eocene sandstone at Rock City area of the park indicate that a major event, such as a large storm, created a stream moving fast enough to carry heavier stones.

If you peer at Rock City's sandstone closely enough, you just might find tiny marine fossils.