What can the Radiolaria tell us?
Radiolaria are still living in the oceans today. Different species live in tropical oceans versus temperate or cold ocean water. By comparing the types of Radiolaria in local chert with modern forms, we know that Franciscan chert contains some tropical and subtropical forms. Based on this observation, it appears that the sediments forming the local chert were deposited far to the south of their current location around San Francisco. Bay Area chert possibly came from the north equatorial upwelling zone, at the latitude of present-day southern Mexico. Scientists also study the various species of Radiolaria that are present in chert deposited at different times in the past. Through these studies, they have developed an evolutionary sequence for different species in the rocks. This evolutionary sequence, or biostratigraphy, is then linked to radiometric dates obtained from associated volcanic rocks. Then geologists can determine the age of the chert. The radiolarian species in the Francisc
