When you need to clear a field, you have to move the rocks out of it; when you need a fence, you use the most cost effective durable materials: two problems with a single solution, field fences.
These are all over Northern California, and parts of Idaho, Nevada, and Oregon. Little to no materials for posts, let alone rails;; barb wire still some years in the future; cattle, sheep, and pig* grazing, plus dry-land grain farming needs all enhanced by removing the surface volcanic rocks that had rained down on the lands...what to do with it? Fence the fields.
Some have gates built here & there; others, it was a matter of unstacking to create a temporary opening, to drive animals from one grazing area to another.
In the 1950s, they were easily seen from roads, and were still in half decent repair; 3-4’ high. By now, from lack of upkeep, having been replaced with wire, or abandoned entirely, they are crumbling.
The picture is a fallen one, and is a poor example, but the clearest photo I could find. Better photos showing extent, but longer so less detailed, shots at the first link above.
*Pig grazing: From the late 19th Century, into the early 20th, pig herds were driven from the northern Sacramento Valley, into Oregon, and into the Sierra foothills, for summer & fall grazing; and back again for the winter & spring.