Chert: The Original San Francisco Rock Star

 

 

By Howard Williams, Veteran MAC Team Member

 

As famous as the City's cable cars and bridges is her unique natural history. Fog shrouded hills and winds sweeping in from the vast Pacific are as familiar to us as the structures built by humans. Though not as famous as the fog and sea, the sedimentary rock known as radiolarian chert is literally what the City is built on. 

Chert is a sedimentary rock found over much of the world but it has a special place - or rather, several special places - in San Francisco. 

 

Chert comes in a variety of forms and colors. Here in the Bay Area, the two most common colors are reddish brown and dark gray. Probably the most familiar example is the outcrop of reddish chert folds on the west side of O'Shaughnessy Boulevard above Glen Canyon

 

 

 

Not as familiar but more impressive is the steep chert massif on the east side of Corona Heights. Rising over fifty feet perpendicularly above a children's playground, this huge gray block gives Corona Heights its other name : Rocky Mountain. This naturally made monument is a textbook example of slickenside, a word that means just what it sounds like : a polished rock surface. Slickensides are caused by the frictional grinding of earthquake activity. As you look at the Corona Heights chert slickenside, you are standing on its matching block; it's just lower and has been covered by soil, plants and a playground.

The chert slickenside on Corona Heights

The most common form of chert in San Francisco is radiolarite which was formed on the bottom of the Pacific eons ago by masses of microscopic skeletons of radiolaria - one celled protozoan similar to the amoeba kids see under the microscope in science class. While they are alive, radiolaria float on the sea. Many eons ago, the skeletons of dead radiolaria drifted down to the ocean floor. Radiolaria skeletons are made of silica, the same material that forms sand, computer chips and breast implants. As the eons passed, astronomical numbers of these microscopic skeletons were compressed by undersea volcanic activity and intense water pressure into massive blocks of chert. Small amounts of iron oxide gave the rusty red color that distinguishes most San Francisco chert.  These massifs were then pushed up thousands of feet by earthquakes to rise above the sea and form the central highlands of what is today San Francisco.

 

Though they are the result of earthquake activity, the City's chert based hills are the safest places to be when the next big one hits.

Fog approaches a chert outcrop on Sunset Heights. 14th Avenue near Ortega looking from the north.

The same outcrop looking from the south.

 

Special thanks to Richard Condrin, BS Geology (San Francisco State University) for his assistance.

 

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CATEGORY: San Francisco