Interesting ones – I've seen some stuff up there too – years ago before I was very interested.... Not sure if it's the same one, but http://www.themodernantiquarian.com/image.php?image_id=19059 is very familiar. I'll have to go have another look – been meaning to go & see the henge for ages anyway!
Looks like a cairn 'base' or cairn circle.
There's something similar in the next field to Appletreewick stone circle. I have a photo somewhere - I'll see if I can find it.
Not doubting you – I 'know' you better than that – is 'Grassington 1' more obviously a circle than it looks in the piccie? These little obscure ones so often are!!!!
BTW – when were you @ Appletreewick – I was up there yesterday & didn't see you (LOL)!!!!
love
Moth