Puzzle of rubbing-stones

Two rubbing stones figured which used to stand beside the Port Elizabeth Museum in Bird Street. About 1.5 m and 90 cm high. Probably collected and donated to the museum by Mr. Katzenellenbogen in about 1910, found on a small hill between Mafeking and Vryburg. An examination of the two stones reveal some interesting effects of the constant rubbing which must have taken place over many years, dolerite being the hard stone that it is. Note that the smaller stone is worn smooth over its whole dome, whereas the top of the larger one still has the rough weathered markings of the original dolerite. This effect suggests that the animals concerned might not have been very large.