the rock that forms the main peak of the island is radiolarian chert underlain by limestone and graywacke sandstone. all these rocks are part of the franciscan assemblage, the same range as albany hill, 3 miles south, and the coyote hills, 25 miles south in hayward. this franciscan formation is derived from sediments laid down in the sea west of san francisco during the late jurassic and cretaceous (150-66 million years ago) and then scraped off onto the edge of the north american plate during the subduction of the farallon plate. the main portion of the island covers an area of approximately , with the tidal lands along the breakwater covering approximately more.