This was used by Mexican miners for milling precious metals ore during the 1800s. Two large stones with flat faces were attached by chains to a horizontal beam. A mule or donkey was hitched to the beam and moved in a circle around and around, dragging the stones over a bed into which pieces of ore were placed. One stone is shown with its smooth face exposed. The finely milled ore was then scooped up with a shovel for further processing to extract gold or silver. A number of these arastres are to be found in the southwestern U.S.