I’ve occasionally stumbled on examples of what are known as the East Bay rock walls or Berkeley mystery walls. No one knows who built them or for what purpose; the consensus is they’re a few hundred years old. "East Bay" and "Berkeley" are actually misnomers: the walls are clustered in numerous locations throughout northern California.
Speculation regarding their origin is boundless:
• “A heretofore unknown, large scale, and distributed culture”
• Lemurians, survivors of the lost continent of Mu
• Indians wanting to corral a vast lost herd of bison that had wandered in from Colorado
• U.S. soldiers during World War 2: the walls were meaningless projects to keep them busy
• Migrant Chinese, perhaps castaways from the great Zheng He fleet of the 15th century
• Chinese laborers of the 19th century, who cleared fields to facilitate farming and ranching
• Owners of the Mexican land grants of the early 19th century
• White ranchers, who used the walls to corral their cattle
• Ohlone Indians, who used them for defense, marking territory, or symbolic purposes
The photo above shows a particularly splendid example of one wall, which I haven't visited. Alice, my Chorkie, and I encountered another such wall on one of our hikes in Sierra Vista Open Space Preserve in November 2015.
It's not particularly impressive. (l) view looking south, (r) location map
The overhead perspective below provides abundant clues regarding what this wall is doing here, and we might even extrapolate the theory to all of the dozens (hundreds?) of such walls in the grassy foothills of northern California.