Bedded Chert at Marin Headlands
September 21, 2014
Shown above is bedded chert rock in the Marin Headlands of northern California, just north of San Francisco. Chert is a silica rock composed of cryptocrystalline quartz, which comes in a variety of forms. This is the same mineral as the familiar quartz crystal, except that the crystals are microscopic. It's formed from the hard shells of microscopic sea creatures (single-celled radiolarians) that lived and died hundreds of millions of years ago. The shells settled to the bottom of the sea in beds hundreds of feet thick and solidified into a very hard silica rock. Later mountain building processes folded and uplifted the layers. Eventually, the rock was exposed by erosion. Different colors are caused by other minerals trapped within the chert. For scale note that the green shrub at far left is about 3 ft (1 m) in width.
Photo Details: Camera: Apple iPhone 5s; Focal Length: 4.1mm (35mm equivalent: 72mm); Digital Zoom: 2.400x; Aperture: f/2.2; Exposure Time: 0.0002 s (1/4566); ISO equiv: 8000.