From the pic, looks like a metate (the grinding basin) and below it the mano (the grinder). Hope I got that right. Very common in agricultural societies as they were used in grinding grains. We found a lot of grinding basins in Western Oklahoma made of sandstone. The flour produced resulted in a lot of wear to the teeth. We never picked very many of them up as they were so common. Every so often we would find one made of granite which came from 75 miles away. These would produce a less grittier flour. It also appeared rather than lug them around the countryside, the people would turn them upside down and leave them at campsites for when they returned. We found a lot by turning over large flat looking rocks. The ones we found dated to about 800 to a 1,000yrs. ago based on association with other archaeological evidence. But I would say they have been in use for thousands of years, or at least since when a people began gathering or farming grains and seeds. Cool find, and keep looking around as these are usually found in association with village sites.
