They way the human brain is wired, you're more likely to see human faces in random texture than you are anything else. But when pushed, you could probably find generic zoomorphs easily too.

There's a sort of face on Long Meg, but you need the light just right, I don't think was done on purpose.

It's a good concise bit of an article.

Terence Meaden claims to have seen 59 (at my last counting) heads within the henge at Avebury; some are natural, others have been worked. He also claims (I think) that if you go amongst the remaining sarsens on the Downs, then you wont find any.