My initial thought was boundary stone, as the boarder between Yorkshire and Derbyshire runs through this area. But after looking at your pics, I'm not so sure, as it seems to show signs of vertical weathering, which means it could have been stood for a very long time... plus the fact that there is no shortage of prehistoric activity in the area.

Very interesting. I was planning a jaunt up to Derwent Edge, the Wheel Stones and Salt Cellar quite soon anyway, so will look out for this and have a good poke around.