Bob Pyle's Studdie might well be a 'natural rock feature' - it's a large sandstone boulder - but it's deemed worthy of Scheduled Monument status. A 'studdie' was a local word for an anvil, and Bob Pyle allegedly a blacksmith who lived in Rothbury in the 19th century*. It's on the western slope of Simonside.
This is all mentioned on the Northumberland National Park website, which also suggests that the boulder could have had significance for those bringing animals up the holloway onto the hilltop. There are a number of Bronze Age cairns around here too.
But an anvil on a hill.. oh how I would like this to belong to someone a bit more legendary and supernatural, with lightning bouncing off it when they thump it. Maybe Mr Pyle was quite a legend. Or maybe he was just the latest person for the anvil to be associated with? (ever hopeful) And might not a duergar have a use for an anvil?