Tilberthwaite is an area in the Lake District fells near Wetherlam with old slate mine workings, and was prospected for copper in the 20th century. It is the prototype for Golden Gulch in Pigeon Post.
AR noted in his diary for 1935: Gnossies to tea. Oscar full of help about the mining (17 February). Then Went up behind Tilberthwaite with Gnossie. Saw several old workings and a suitable bit of country for my story (27 March). From 1928 Oscar employed an old miner John (Willie) Shaw (the original for Slater Bob); first, with little return, looking for copper, and from 1933 quarrying slate at Horse Crag, Tilberthwaite below Wetherham (NBUS page 88).
Christina Hardyment investigated Tilberthwaite Deep Level and Birk House Mine. She said that with many vertical shafts as well as horizontal levels; Wetherlam is no place for small children or inquisitive dogs. Tilberthwaite Deep (Old) Level linked the copper mines to Penny Rigg Quarry, a tunnel over 3,000 feet long driven through the hillside, similar to the tunnel rashly used by the "hurrying moles".