This carved faces is an equator-seeker, please click here to see others
Interestingly enough this face on 13a is paired with a blocky face on neighbouring Stone 13b. I say they are paired because although these sculptures are different in style they both appear to be studying the same part of the southern sky (click here and scroll down to see the information about what I describe as "Face A").
Coincidentally appearing to balance on the carving's nose is the famous "Kemble's Cascade" stellar asterism in the constellation of Camelopardalis, see the larger version of this image for a clearer view.
Image copyright David Baldwin Night Photography