11.1: A Complicated Land

There’s more complexity in the Coast Ranges than can be described in one book, much less this one brief chapter, but one place to start is to understand that, along most of their length, the Coast Ranges are the place where the North American plate and the Pacific plate meet. This one seemingly-simple statement explains a lot about the Coast Ranges. But fully understanding what this means takes some unpacking.

The Coast Ranges were first formally mapped in 1895 by Professor Andrew Cowper Lawson of UC Berkeley, a distinguished mineralogist known today to geology students as the guy with the beard of unusual and splendid magnificence. The hirsute Lawson understood the importance of field work–indeed, he founded the first systematic field geology course in the west–and during his explorations of the Coast Ranges he accidentally discovered the San Andreas fault in a linear valley south of San Francisco. At the time Lawson had no idea that the San Andreas’s offsets were anything more than a local disruption. That changed in 1906, and Lawson headed a massive research effort to examine this fault that had ruptured such a great distance and so catastrophically tore the state apart. However, Lawson came from a time before plate tectonic theory; in 1906 he could describe what happened geologically, but not why.

Though Lawson would not live to see the advent and acceptance of plate tectonic theory in the late 1960s, today we know that comprehending the California Coast Ranges is impossible without plate tectonic theory. Just as biology prior to Darwin’s theory of evolution has been described as mere butterfly collecting–assembling organisms without understanding how they came to be–geology before plate tectonics was conceptually hobbled and unable to explain regions such as the Coast Ranges.

Why do we find pillow basalts–lava that can only form underwater–in contact with metamorphic rocks such as eclogites that have been subjected to brutal extremes of heat and pressure, and both of these right next to unaltered sandstones? If all this formed in place, when these eclogites were metamorphosed, why weren’t the pillow basalts and the sandstones also metamorphosed? These sorts of Coast Range juxtapositions make no sense–until one understands that tectonics has brought all these disparate pieces together from far away.

There are also complications of time and order.

In certain places in the world–Grand Canyon, for example–rock formations obediently accede with geologist’s wishes by forming one atop the other, with the occasional fault or fold distorting otherwise relatively flat sedimentary units. It is possible to stand on the South Rim of Grand Canyon and look down to the river and see the Tapeats Sandstone neatly lying atop even older Vishnu Schist, and the Bright Angel Shale atop the Tapeats, and the Muav Limestone atop the Bright Angel, on and on and on, right up to the younger Kaibab Limestone on which visitors stand. In such situations, we can easily understand that the layers are in order, with the oldest at the bottom and the youngest at the top.

California just isn’t like that.

How do you explain how old a rock assemblage is when most of it formed far from California, then was brought in by tectonics to collide with and stick onto California, a process that altered and metamorphosed the rocks? What’s the proper age of the assemblage? When it first formed in its original state? When it moved and accreted onto California? When metamorphism so transformed the original rocks that they are unrecognizable? There is not an easy answer here.

Further complicating the age of rock groupings, several of the major groups date from the same time range. Geology uses a fancy word for this: “coeval,” meaning forming at the same time. Because of the devilish difficulties coeval rocks pose for interpretation, we might as well just relabel them as “evil.”