The big set of knockers, Mountain View Cemetery

biggest knocker

Just below the highest hill in the cemetery, across a flat space south of the utility yard, is the best bedrock outcrop in the whole Mountain View property. This shot is from the south end looking toward the utility yard; red chert is in the foreground and other Franciscan rocks lie behind. I suspect that it was once a free-standing ridge that has been filled in on the east (uphill) side. The west side is a wall of trees, some of them growing right out of the rock, that hides everything pretty well.

Rocks of the Franciscan Complex, to remind everyone, include red chert, light-gray coarse-grained sandstone, dark shale, and dark volcanic rocks with various degrees of metamorphism. The volcanic rocks came first, formed at a deep-sea spreading ridge. Deep-sea ooze made of siliceous microfossils settled on the volcanics and became the chert. As the whole seafloor assemblage approached North America, sediment from the continent cascaded down submarine canyons and later turned to sandstone and shale.

All of these entered a tectonic subduction zone, marked by a deep-sea trench like those off Japan today, and the whole assemblage was squeezed, heat-treated, crumpled and plastered against the prow of the North American continent. The different rock types, ranging in age between about 150 million and 60 million years (Jurassic to Paleogene), were churned into an intricate mixture called mélange. Chunks of the harder rocks float in a scaly matrix of soft shale and tend to emerge above ground as the rocks erode into soil. Those chunks, not quite bedrock and not quite boulders, are what generations of California geologists have called “knockers.”

Later, sideways movements along the wide San Andreas fault complex tore up and rearranged this complex of rocks even further. Today Franciscan mélange is found in the Coast Range from south of San Luis Obispo all the way up to Cape Mendocino.

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