sand - coarse sand composed of polished chert grains derived from the Franciscan complex - set of two 2-ounce jars

Sand is derived from whatever material is available. In this case, the source of the chert pebbles is banded ribbon chert which alternates with thin shale beds of the Franciscan Complex in the Marin Headlands. The Franciscan is a body of rocks scraped off the Pacific Plate as it subducted under the North American Plate. The chert underlies roughly half of the rocks in the Marin Headlands, which support the north end of the Golden Gate Bridge. Resistant to weathering, chert grains are concentrated at Rodeo Beach by the waves, where it forms a barrier bar with a lagoon behind. Traces of iron in the chert are oxidized in different amounts to produce a spectrum of brown, red, green, yellow and black pebbles.

The lack of land-derived sediments in the chert-shale sequence indicates that it was formed far offshore. The chert contains the siliceous skeletons of radiolarians that lived in the equatorial Pacific. Oxidized radiolarian cherts are associated with high-productivity upwelling zones north and south of the equator, suggesting that the sediments forming these rocks were deposited near the equator. The oldest radiolarian fossils in the cherts lived about 200 million years ago. The youngest species lived about 100 million years ago, so the cherts represent 100 million years of open water sedimentation of radiolarian skeletons and form one of the longest stratigraphic sequences of chert in the world. 

Set of two 2-ounce jars.

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