Claremont Ribbon Chert EarthCache
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The real Claremont chert
By Andrew
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?
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