The Twin Tanks are a couple of natural tanks enhanced by cattle ranchers to hold more water with concrete dams. The southern dam is concrete with rock mixed in, and the northern one is plain concrete. This one also has an inscription of "C. O. Barker" in the corner.
In the late 1890s and early 1900s, Barker was a cattle rancher who improved many of the natural tanks in what is now Joshua Tree National Park, including building the more famous Barker Dam. You may be surprised to learn that cattle ranching would be profitable here in this arid desert, but rainfall was more plentiful a century ago.
There are a few petroglyphs and pictographs out in this area. They are faint and difficult to find. A set of cupules is below the South Dam.