Not all of the figures identified as prehistoric art are in fact human made. An example given by archaeologists Paul Bahn and Paul Pettitt is the 'horse-head', Which they say is ""highly visible and resembles a heavily maned horse-head... lacks any trace of work: it is a combination of erosion, black stains for the head, and natural burrow cast reliefs for the mane." Others are a 'bison-head' which they think may be natural and a 'bear' image which "lacks any evidence of human work." Notwithstanding they believe that more figures may be discovered in the future.