The preferred term is “cupstone” since you are quite right that there’s a lot of things we don’t now about their usage, but the evidence says that they are not statistically associated with the kitchen portion of middens. They certainly didn’t have a single usage and were likely used for preparation of medicines, paints and pigments as well as for cracking nuts. Acorns in particular were ground as well as hulled, to produce a thickener for soups and gruels.
Other probable uses include the base socket or palm-protector of a fire-drill (when using the stick and bow method) and as a stabilising or securing socket when working arrowshafts, spearshafts, poles and spindles. The pattern, size and number of cups varies enormously and stones frequently show signs of grinding marks (both linear and circular) as well as impact damage on one or more surfaces. Some have drainage channels carved from the cup to the edge of the stone, or rudimentary spouts, and very clearly were used to process liquids. Sometimes, the cup has been hollowed out by back-carving to become the major part of a sphere.
It’s odd that when there are multiple cups, they are frequently almost the same size and so clearly not intended for nuts of different sizes. Also the closeness of the cups does not allow for multiple users performing the same operation crowded round a single stone… not even a biggish one. Different paint colours? Separate processing of medicinal substances which are then mixed in particular proportions? The latter might possibly explain overlapping cups. Heaven knows what waist-high cavities on a cave wall might have been used for.
In an effort to shed more light on this area, almost a hundred cupstones recovered from 2 adjacent semi-coastal sites in San Luis Obispo were subjected to detailed scrutiny in an attempt to associate them with particular usage by Breschini & Haversat (1988-1993). Shell debris from the sites puts the radiocarbon date between about 3,500 to 4,900 years old (uncalibrated).
Most of the stones were relatively small and some of the cups were quite shallow, suggesting that they might have just performed as a finger or thumb grip on a hammerstone rather than being an impact anvil. The possible use in pounding tasks was supported by wear and impact damage on some of the edges of the stones - not on the flat surface where the cups had been ground out. Some of the cups were positioned acentrally and some of the stones had double-cups – mostly one on each of the two opposing surfaces, but sometimes with both cups on the same surface.
Even within food preparation, usage was not confined to nuts and acorns, the residues of which are frequently found with larger, non-portable stones. Breschini & Haversat have subjected 22 of the stones – both double and single cupped – to testing for protein residues using anti-sera from animal species likely to have been used for food or other resources. The only reactions observed were that 3 of the stones tested positive for trout (or a fish of the salmonid family), 5 tested positive for clam and one tested positive for both of these. Bear, cat, chicken, deer, dog, guinea-pig, mouse, rabbit, shark and sturgeon were all negative.
