Turtle Rock. Tiburon Peninsula, CA. This block, and nearby Split Rock, stand out of their soft serpentinite rock surroundings (Franciscan Complex). Called a "knocker" by some geologists, it is a high-pressure, low-temperature metamorphic rock, interpreted to have been formed in a subduction zone, where ocean crust of the proto-Pacific plate dived under the North American plate. Turtle and Split rocks are dated at ~ 150 million years, and are composed of the minerals glaucophane, lawsonite, jadeite, and chlorite. I have spent many wonderful times after work in SF, bouldering on this rock. California poppies grow in magnesium- and iron-rich soils, which are inhospitable to many plants.