What can the fossil shells tell us?
Scientists have studied foraminifera found in sedimentary rocks. After identifying these fossils, geologists develop an evolutionary sequence of the species. This sequence, called a biostratigraphy, helps geologists determine the age of the rocks. The limestone in the Marin Headlands is too metamorphosed and recrystallized to contain original fossil shells, but other local Franciscan limestone contains fossils that tell us it was deposited from about 125 million ago to about 90 million years ago.
