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.


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