As of the United States Census , the population was 1,, The county's name refers to its position on the other side of the bay from San Francisco. In prehistoric times, particularly the Miocene epoch , portions of the landforms now in the area then marshy and grassy savanna were populated by a wide range of now extinct mammals , known in modern times by the fossil remains excavated in the southern part of the county.
In the northern part of the county, significant coal and sand deposits were formed in even earlier geologic eras. Other areas of the county have ridges exposing ancient but intact not fossilized seashells, embedded in sandstone layers alternating with limestone. Layers of volcanic ash ejected from geologically recent but now extinct volcanoes , compacted and now tilted by compressive forces, may be seen at the site of some road excavations.
This county is an agglomeration of several distinct geologic terranes , as is most of the greater San Francisco Bay Area , which is one of the most geologically complex regions in the world. The great local mountain Mount Diablo has been formed and continues to be elevated by compressive forces resulting from the action of plate tectonics and at its upper reaches presents ancient seabed rocks scraped from distant oceanic sedimentation locations and accumulated and lifted by these great forces.
Younger deposits at middle altitudes include pillow lavas , the product of undersea volcanic eruptions. Early contact was fleeting and mostly peaceful, however, in the late s, Spanish cattle were grazed the territory of the Saklan and their neighbors. The Saklans were one of the few groups who mounted resistance against the efforts of the Spanish to convert them to Christianity. Christian natives were sent to the Saklans to convince them to give up their culture and come to Mission San Jose; they were killed by the Saklans who wanted nothing to do with the Spanish way of life.
A punitive expedition was sent to the area in to repress the Saklans, and a battle was fought in modern-day Lafayette. In the early s, many Saklan fled to the mission as a result of societal collapse from disease.
