Franciscan Greywacke/Shales
Composed primarily of quartz and plagioclase feldspar with a chlorite
mica
matrix, graywacke constitutes ninety percent of the rock in
California's
Coast Range. Here in Marin, these muddy sandstones that geologists
first
called greywackes are layered with mud-rich shales. These layers form
the
foundation of the Tiburon Peninsula and are seen being interpreted by
Jim
Locke as to the "younging direction" within the layered and tilted
sequence
in this photo
on the road cut opposite Triangle
Marsh and just west of the Corte Madera entrance to the Ring
Mountain
Preserve. The results of his instruction can be seen in this
photo of some historical geology students putting the instruction
to
work. Are they confused? A closeup view shows that while the layers
were
laid down horizontally they are now steeply inclined but were also
folded
(isoclinally) and provide graphic evidence of the forces that were
generated while the plate tectonics collision was "bulldozing" these
rocks
up into mountains along the California coast.
These layers currently lie at the base of Ring Mountain. However, at
the time of their deposition by turbidity
currents one hundred to two hundred million years ago, they
actually
sat above the chemically distinct older rocks which now rest at the top
of the mountain. An explanation for this upside down arrangement is
discussed
in the section on California's
serpentine.
The alternating layers of light graywacke and dark shale are
identical
to sedimentary deposits which marine geologists observe forming on the
ocean floors today. Their research on these modern deposits and
comparison
with ancient deposits enlightens our view of the world that existed
when
these thin slabs were individual grains of sand and mud, billowing
along
the floor of an ancient sea and leaving their traces
in the rock record. A example of a modern environment in which the
Greywacke/shale layers were formed is seen in this seismic
profile collected during the voyage of the USNS Kane in 1968.
Other related Greywacke sites
Here's
an interesting technical article from Australia on a greywacke that was
converted
to a Granite.
