Claremont Ribbon Chert
In California, United States
Size:  (not chosen)
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The real Claremont chert
The Claremont chert actually has the official name Claremont Shale. The rock unit was first named by Andrew Lawson, UC Berkeley’s memorable professor of geology who masterfully conducted and wrote up the scientific studies of the 1906 earthquake. (Lawson has the mineral lawsonite named for him, too). Lawson named the Claremont Shale because the unit is predominantly shale, even though in Claremont Canyon, as in the rest of Oakland, it’s quite cherty. When geologists name a rock unit for the first time, they designate a type locality and take the name from that. They also designate a type section, a specific place where anyone can visit and check the definitive example of the XYZ Formation. They draw a detailed stratigraphic column from the type section—it’s sort of like an engineering drawing, very formal. And they publish a detailed description of the unit and the stratigraphic column in a reputable journal or government-issued publication. The type section is supposed to display the top and the bottom of the formation and, ideally, everything between. The US Geological Survey has a panel of experts who do nothing but keep track of geologic names, both for rock formations and for the Eocene/Jurassic/etc. geologic time units. So Claremont Canyon is the type locality for the Claremont Shale. Lawson’s original description is in US Geological Survey Geological Atlas Folio 193, published in 1914. And this exposure of the chert along upper Claremont Avenue must make up part of Lawson’s stratigraphic column in that old atlas.
The above information can be found at: Oakland Geology
Most chert forms when microcrystals of silicon dioxide grow within soft sediments that will become limestone or chalk. In these sediments, enormous numbers of silicon dioxide microcrystals grow into irregularly-shaped nodules or concretions as dissolved silica is transported to the formation site by the movement of ground water. If the nodules or concretions are numerous they can enlarge and merge with one another to form a nearly continuous layer of chert within the sediment mass. Chert formed in this manner is a chemical sedimentary rock. Much of the Claremont Formation consists of thin bands of whitish chert. Here a sequence of layered rocks representing marine, alluvial, and volcanic environments are exposed. These rocks were more or less flat lying when they formed, 9 to 16 million years ago. As the East Bay Hills were pushed up, the rock layers were folded and faulted to their present positions. This rock is similar in type and age to the Monterey Formation found at Point Reyes and on the Peninsula, but it was deposited in a different basin. The Claremont Formation tells us that about 14 million years ago the area in which it formed was still marine.
The formation we are examining is located on Claremont Avenue in the Berkeley California hills. There is parking for several vehicles off the road across the Avenue from the formation. Please do not block the gate.
While you are there, try to determine if this is chert or shale or some of each. Also, think about the force or forces which caused the formation to be in its present orientation.
To log this cache send an email to me with your answers to the following two questions. Do not post your answers in your log.
Would you call this Claremont chert or Claremont shale?
What force or forces were at work to move the horizontal layers into the present orientation?
(No hints available.)
Last Updated: on 9/29/2015 8:18:33 PM Pacific Daylight Time (3:18 AM GMT)
Coordinates are in the WGS84 datum