An arrastra is a simple and primitive gold milling device. Heavy stones are dragged over gold ore to crush it. Mercury and water is added to the floor of the arrastra to create an amalgam and capture the gold.
There is somewhat of a mystery about who built this arrastra and used it. There is a small incline shaft up on the hillside above the arrastra and there are a couple of prospects in the area. The arrastra could have been built to mill ore from these prospects. Perhaps the ore played out too quickly and the miners moved on before using the arrastra heavily.
It is also possible that this was a custom mill for other mines in the area, maybe centrally located to serve some of the other mines?
The location in the backcountry of Joshua Tree National Park is also a bit of a mystery too because there is no good source of water nearby. It would have needed to be brought in for the arrastra to work.