The History and Archaeology of Yerba Buena Island - GeologyThe Formation of Yerba Buena Island
and the San Francisco Bay
22,000 YEARS AGO (The last glacial maximum. During the Pleistocene Epoch.)
Glaciers from the last ice age covered much of the Sierra Nevada Mountains, and the Pacific Ocean was about 400 feet lower than today. San Francisco Bay was a dry valley. At that time you could walk to what are now the Farallon Islands, about 30 miles offshore.
18,000 - 11,500 YEARS AGO
The glaciers started to retreat and the sea level began to rise. The rising water submerged and transformed a landscape that was populated by mammoth, bison, ground sloth, and camel.
11,500 - 5,000 YEARS AGO (The beginning of the Holocene Epoch.)
The rising sea flooded the area that became San Pablo and San Francisco bays, and turned Yerba Buena into an island of about 145 acres plus tidelands. Yerba Buena is an unusual assemblage of rocks known as the Franciscan Formation-a mix of volcanic rocks, deep-sea cherts, sedimentary sandstone and shales, limestone, and metamorphic rocks, including serpentine-which was formed from rocks that were once part of the earth's mantle.