Hikers in San Francisco know them well: miles of stacked stones spanning portions of the rugged hills overlooking the East Bay. The crude walls have been compared to Easter Island and Stonehenge because like those two ancient sites, their origins are shrouded in mystery. No historical documents exist to shed light on who built them, or when, and as a naturalist told The Monthly in 2014, "The mystery walls never attracted serious scholarship, so all we have is in the realm of people spinning ideas." Now, local archaeologist Jeffrey Fentress is measuring and mapping them so as to gain them entry into the state archive, which would lend protection from things like development. In light of Fentress' effort, the Mercury News takes a look at what we know, or think we know, about the walls.
At the turn of the 20th century, scholars speculated the 3- to 4-foot-tall mortar-free walls, which do not run continuously, were built by ancient civilizations, or by the Mongolians. A recent testing of lichen growing on the rocks suggests otherwise. While not a perfect dating technique, it places them in the 1850-1880 period. Modern experts' best guess is they contained the cattle of post-Gold Rush European immigrants, though there's no consensus on which country those immigrants hailed from—or whom they may have tasked with building them (Mexicans? The Chinese? Native Americans?) But those unknown builders are forever captured in the walls, says Fentress: "They are essentially the archaeology of the working class, the common people who came here and made a living."