NEHL - Cause Papers regarding tithes & coal mining around Hexham detail coal mines to the east of Fallowfield Dene in the late 16th century. There is not an exact location noted, however a number of old ventilation shafts and entry shafts are noted on 19th century Ordnance Survey maps at Fallowfield Fell. There is a chance these workings are related. The Coal Authority also noted dozens of mine entries in this area.

Thanks to Dave King (@n_docs) for the excellent info.