An arrastra is a simple and primitive gold milling device. Heavy stones are dragged over gold ore to crush it. Mercury and water are 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 it was 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.
The design itself of the arrastra is also rare because of that wagon wheel sitting on top. It is a "wagon wheel" arrastra, meaning that a wagon wheel is attached to the central axis to drive the drag stones. This type of arrangement is quite unusual and very few, if any, are left in the wild. This is the only known remaining example of this type in the backcountry of a National Park.
Typically, arrastras were driven by walking a mule or horse around the perimeter. The wagon wheel design was to accommodate a belt for an engine, in this case a four horsepower International gasoline engine (long since removed), to power the arrastra. Having two wheels on the arrastra allowed for switching the rotation speed of the crushing stones. The larger wheel was probably added later to reduce the revolutions per minute and slow the crushing down to a desired speed.
The gasoline engine suggests a date after 1900s for the arrastra but probably before the end of the Great Depression in the 1930s. The Monument was created in 1936 and no new mining claims would have been allowed after that date; work on existing claims could continue. So, it is likely the arrastra was built before then. There is a small sign at the site explaining the workings of the arrastra.