The Falls Trail loop is one of Mount Diablo's most popular hikes. In addition to impressive cascades and wildflower displays, the walk provides an introduction to the fascinating geology of Mount Diablo. The loop is six miles long, with a total elevation gain of 1,300 feet. Portions of the route are rugged, so it is advisable to wear lug soled boots. The trail head is at the end of the Regency Drive cul-de-sac in Clayton. The first portion of the trail follows Donner Canyon Road south along Donner Creek. Although Donner Canyon Road is wide and well-graded, this first part of the hike is often challenging after a heavy rain, because of the peculiar geology underlying the lower portion of Donner Canyon.

​

These rocks belong to the 140 million year old Knoxville Formation, part of the Great Valley Sequence. These Mesozoic (Dinosaur) age rocks are composed of material weathered off the ancestral Sierra Nevada mountains and deposited into an ancient sea. The Knoxville Formation includes units of mudstone that are rich in clay minerals. Clay is extremely porous and readily incorporates water between flat, platy grains. Although porous, clay is nearly impermeable, and therein lies the problem. The "fat" clusters of water molecules cannot squeeze through tiny pore spaces between the clay grains nor overcome the electrostatic attraction exerted by the platelets. The water stays trapped in the pore spaces. The applied result of these hydrogeologic properties is a muddy goop that sticks to the bottom of your boots as you slog up the trail.

About one mile up Donner Canyon, you will encounter the old Donner Cabin site on your left. Once a park residence, the cabin burned down several years ago. Beyond the cabin site, the trail climbs steeply for a short distance. As you climb, keep a sharp eye out for gray-brown chunks of sandy limestone in the soil. These chunks represent concretions encased in the Knoxville mudstone.

Continue up the Donner Canyon 3/4 of a mile to the junction with the Meridian Ridge Trail. In springtime, look for the California State Flower, the Poppy, and the California State Bird, the Quail. Pause at the trail junction and examine California's State Rock, Serpentinite.

Serpentinite is a metamorphic rock, created by the hydrothermal alteration by seawater of peridotite, the rock which comprises the mantle of the earth. The mantle is the layer of the earth's interior that lies between the relatively thin crust and the earth's core. Serpentinite is an ultramafic rock, rich in the elements iron and magnesium. Serpentinite also contains concentrations of nickel and chromium. This unusual mineralogy results in a distinctive plant community dominated by scrub oak, manzanita, and digger pine. Although only 1% of California is underlain by soil derived from serpentinite, fully 10% of California's endemic plant species grow in a serpentinite substrate.

Turn right (west) at the junction of the Donner Canyon Road and Meridian Ridge Road, proceed uphill 1/10 of a mile on the Meridian Ridge Road and turn left at the intersection with the single-track Middle Trail. Finding the route to the falls was once a formidable task, but strategic intersections have been well marked through the determined efforts of Harvey Brossler, Paul Bennett, and other volunteers as part of an ongoing MDIA sponsored trail signage project. New trail posts have been erected throughout the park and signs are being affixed to the posts. Ranger Jay Sherman and former Ranger Burt Bogardas have also been instrumental in the project.

The Middle Trail winds up the serpentinite hillside with serpentine twists and turns for 1/2 mile, to the junction with the Falls Trail. At the intersection, take the Falls Trail, which branches to the left. After another 1/4 mile climb, the trail dips to cross a creek. During the cold winter months, this creek crossing provides a truly remarkable spectacle. Here, thousands of ladybird beetles cover every rock, tree and shrub. "Ladybugs" spend the winter in semi-hibernation on Mount Diablo. When the weather warms, the ladybugs mate and fly off on prevailing easterly winds to coastal valleys where they will lay eggs to produce the next generation of beetles.

As you continue up the Falls Trail, notice that the mottled serpentinite is replaced by a dark colored rock. This is basalt, an igneous rock erupted from volcanic vents on the ocean floor at tectonic plate spreading centers. These are the oldest rocks on the mountain. Radiometric and paleomagnetic studies indicate that the basalt is 190 million years old and formed thousands of miles out in the Pacific. These rocks were rafted here from their remote origin at spreading oceanic ridges by movements of the Earth's plates. The basalt is part of the Franciscan assemblage of rocks. 

The basalt on Mount Diablo is sometimes referred to as pillow lava. Molten rock that erupts underwater cools quickly on the outside while the inside of the flow keeps moving, pushing the mass into a tube or pillow shape. These pillow structures are especially conspicuous on the cliffs surrounding the falls. These rocks are also called greenstone because the erupting lava reacts with seawater to form new minerals, such as chlorite, which give the basalt a greenish cast. 

Finally, you will reach the Falls about 1/2 mile beyond "ladybug cove". Notice that the water flows over and through the basalt. Freshly formed basalt is generally not a good aquifer, but the rocks on Mount Diablo have been highly fractured over the millennia and so are able to store and transmit water to produce the seasonal waterfalls.

Continue north along the Falls Trail. The fantastically contorted red rock is Franciscan chert. Chert is a biogenic sedimentary rock, composed of the recrystallized skeletons of single celled creatures called radiolarians. Radiolarians look like "amoebas in glass houses". These planktonic organisms live in the warm surface waters of tropical oceans and build their skeletons ("tests") out of silica. After radiolarians die, their tests slowly settle to the frigid bottom waters of the ocean floor to form a siliceous ooze. The ooze is buried, compressed, and over time, "lithifies" to form solid chert. 

Notice that the chert is in discrete layers, with bands of shale (composed of volcanic ash and fine silt blown from land) spaced in between ribbons of chert. The bedding develops when the soft ooze hardens to rock. In a process called diagenesis, the molecules of silica segregate themselves from the shale to form separate chert-shale couplets.

Examine the intricate chevron folds. These folds develop during plate subduction, long after the ooze has hardened into solid rock. This is an example of "uniform flow" or "ductile deformation", in which solid rock deep underground is shaped under conditions of high confining pressure, high temperature, but low directed stress. 

The trail continues to climb briefly beyond the falls, then descends to meet Cardinet Oaks Road in 1/2 mile. As you descend, notice that the vegetation growing on the chert soil is rather sparse. In contrast to the Knoxville formation soil, soil derived from chert is both porous and permeable. This gives the soil a poor water holding capacity and serves as a limiting factor on the plant life.

Walk down Cardinet Oaks Road and cross Donner Creek in .4 of a mile. As you cross the creek, see if you can identify the Franciscan cobbles and boulders that line the creek bed. Continue .1 miles to the junction of Donner Canyon Road, turn right, and return to the trail head.

Thanks to Karen Synowiec and Roi Peers for their many helpful suggestions regarding the content and presentation of this article.

Karen Synowiec is a professional geologist and MDIA hike leader. 

Doris Sloan, MDIA advisor, is Professor Emeritus of Environmental Studies at U.C. Berkeley. She teaches a California Geology class for the general public each spring through U.C. Extension.