This photograph, taken from Cerro San Luis, shows Bishop Peak, Cerro Romauldo, and in the misty distance, Hollister Peak. The hills on the extreme left margin of the picture are the serpentinite ridge that backs Cuesta College, and the hills of the extreme right right margin are the western portion of West Cuesta Ridge. One of the obvious visual features is the dark chaparral scrub of the peaks, and the brown of the late spring grasses on the flanks of the peaks (grasses are still green as we go to press). The serpentinite is a slightly different shade, while the West Cuesta Ridge’s distant hills also reflect a dark ridge line and brown flanks.

What you are seeing is a display of the underlying geology manifested in the soils that lie above the bedrock. The browning grasslands are floored with ‘melange’ of the Franciscan Formation, a unit composed of sheared shale and bits and pieces of the ancient seafloor that were squished into a subduction zone… kinda like what happens if you try and push a slice of pizza under a door. (hint… try this experiment.. you will need a nearby dog). The soils derived from this mess include a number of clay minerals that are highly expansive, increasing in volume by as much as 40% when wetted, and shrinking and forming deep surface contraction cracks. If you have walked Laguna Lake Park you will have seen the cracks. The cracks allow deep drying of the soil profile in the summer, and will fracture plant roots as they develop. As a result, the melange is covered with annual plants, especially introduced European grasses (California originally didn’t have all the springtime bright green grassland).

The Morros themselves are about 50 million years younger than the melanges, and are the lava plumbing stubs for volcanoes that have long-ago been stripped away by erosion. Formed when an ocean ridge system collided with the edge of North America and the San Andreas fault system was starting, the lava had a composition close to that of granite, and weathers into a thin soil that is not expansive. This enables perennial plants to take hold, and as the bedrock is fractured, allows shrub roots to penetrate fairly deeply into the ground.

poly serpentinite

This photo is a pre-fire Google Earth oblique of the serpentinite ridge behind Cal Poly, showing (1) the nearly bare rock, nutrient-poor serpentinite outcrops on the ridge line, (2) green patches of black sage on volcanic rock of the Franciscan Formation at mid-slope (very large blocks of old volcanic seafloor) that forms thick soil, and (3) melange grasslands at the base of the slope.

-David Chipping