What does Chert look like?
In San Francisco, most of the Chert is a rust-red color, as it was colored by the dust blown from the continent. The chert is made up of many layers, each about one to two inches thick. These bands of red chert are spectacularly folded and you can really see why it’s called “ribbon” chert. In between each red ribbon of chert you will notice a thin layer of fine-grained, dark gray shale. This shale probably came from turbidity currents, was mixed up into the radiolarian ooze, and separated out as the chert underwent diagenesis–moderate temperature and pressure conditions that cause chemical changes in a rock without thoroughly metamorphosing it. Geologist actually are not entirely certain where this shale came from or why the “ribbons” are so uniform in thickness. Take a high-powered (10x or 20x) hand lens with you, wet a piece of chert, and hold it in the sun. You may see a small translucent-gray radiolaria that sacrificed its’ life to bring you hills. How did it get on top of our hil
