Dressed stones on Bridestones Moor

A bit of a mystery. Bridestones Moor is unmanaged diverse heather moorland, a National Trust property, bisected by a steep griff or valley along the edge of which are the Bridestones, calcareous sandstone towers weathered into surreal shapes. There is no other rock exposed on the moor, no scattered boulders. Limestone was quarried in a small pocket in the 19th century but apart from the lime-rich pasture, no trace of the quarry remains. Yet there is a cluster of half a dozen stones, several of which appear to be dressed or worked. Now I’m no geologist but the stones seem to me to be a different rock to what I remember of the Bridestones. More grey and coarser. No building is indicated in the vicinity of old maps. So how did they get here? Answers on a postcard, please.