The Chert Wall, by Martin Holden

The outcropping of the Claremont Chert in the middle of Claremont Canyon (the “Chert Wall”) is one of the best exposures of this interesting geological formation. According to rumor, stone was quarried here in the 19th and early 20th Century for use as roadbed fill under what is now Claremont Avenue. Currently, the Chert Wall is part of the University of California’s Ecological Study Area. It provides an accessible place to observe the sage scrub native to the sunny southern exposure of Claremont Canyon, a flora which has been largely displaced by eucalyptus and other exotics elsewhere in the Canyon.

How was it formed? The Claremont Chert is a marine sedimentary rock, originally deposited in a hemipelagic environment. This term refers to depositional basins which are neither near-shore nor in the deep sea. Modern examples of this type of depositional environment would be the basins between the Channel Islands, off of the Southern California coast. The much larger Monterey Formation is another example of a hemipelagic deposit. Both of these formations consist of chert, a hard, finegrained silica-rich rock, plus more friable shales and mudstones.

What is it made of? The main components of the Claremont Chert are the siliceous skeletons of microscopic marine creatures, primarily radiolarians. The interbedded shales are composed of clays derived from airborne dust particles, plus fish scales and other detritus. There are also occasional layers of yellowish dolomite, probably derived from calcareous microfossils such as foraminifers.

How old is it? The Claremont Chert was deposited in the middle to late Miocene approximately 13 million years ago. This is the period when the Earth began to look truly “modern.” Along with the formation of the Antarctic ice cap, the Miocene saw the expansion of kelp forests and grasslands, and the diversification of the mammals.

Why is it layered? The origin of rhythmic bedding is a source of controversy. The layers do not represent annual variations, like tree rings. The couplets that define each layer (consisting of about 10 cm of chert and a thin 1-2 cm band of shale) were actually deposited over a period of a few thousand years each, at least. Therefore, the bedding must represent some variation in deposition that occurs over this long time scale. The most likely explanation is that the layers were formed by changes in ocean fertility and/or wind deposition, relating to periodic climate changes. These changes are probably associated with regular variations in the Earth’s orientation as it orbits the sun (the “Milankovich Effect”).

Why are the layers vertical? The steeply-dipping strata seen in of the Claremont Chert result from strikeslip and reverse faulting associated with the Hayward and related, smaller faults. This deformation began in the late Miocene, and is associated with the movement of the North American Plate relative to the Pacific Plate. Needless to say, this process is still occurring.

What other rocks are found in Claremont Canyon? The oldest rocks in the Canyon are the Leona Rhyolite and the Cretaceous Oakland Sandstone, which can be seen on the Ridge Trail. These Mesozoic rocks are overlain by the Claremont Chert, which is itself overlain by the late Miocene Orinda Formation, a riverine deposit. The youngest rocks are the basalts of the Moraga Formation, which are exposed in the road-cuts of Grizzly Peak Blvd.