At this juncture, I'd like to point out that faking CnRs is quite easy if you can be arsed to spend a few weeks weathering them artificially. If anyone feels like starting a line up on ebay, flogging genuinefakeprehistoricmodern rock art, I think there'd be a market, especially in California. Postage costs for large boulders to be sent across the pond might be a problem though ;)