When they get that big they start to fall into the Bullaun stone catagory. That's another 'out of Ireland' one to add to my list. Irish Archaeos tend to see them as an Irish thing - actually, that's not quite true ... most Irish archaeos don't give them a second thought! Most reckon they are Medieval at best. A few will allow them to be Iron Age. I am the first to propose a Neolithic origin for the Irish ones and the first to really start looking outside of Ireland.

Perhaps you need a Megalithic Ruler - easily made with a bit of cardboard and a felt-tip pen. An M.I. being about 0.815", or the distance between the two creases on your second finger.

Those are excellent pictures of the Devil's Arrows - have you seen Hob's picture of Guinevere's Chair ? http://www.themodernantiquarian.com/post/36817

There are lots of basins this size on stones on the moors. I thought anything that large was considered to be natural weathering?

I'm confused!
When is a basin considered too large to be a cup? And what are the tell-tale signs that it's a cup, formed by man, or a natural process of weathering?!

Cheers
Dave