I agree with the county archaeologist on the medieval thing, by the way, although I couldn't narrow it down to any more exact period than that. They've got a medieval, not a prehistoric, feel to them. Not very empirical, I know. I think they're hugely relevant to the north's prehistory, though, in that many of them are on sites that show all the signs of having been significant in ancient times - like on Fendrith Hill, where the currick is perhaps only fifty yards away from the cup-marks.
Even though I think them to be medieval, for the most part (Shorngate Cross currick - I think that's its name - on the Rookhope-Allenheads road was built last century, for example), I think there can be little doubt that they are heathen. I accept that mediaeval heathenism is probably beyond the remit of this site, though.