"Coal is wrought in this parish [Thornley] by the Weardale Iron and Coal Company at their West Thornley colliery. Here the same seams are worked as in their Black Prince colliery at Tow Law [Ballarat, Five Quarter, Top Coal, Main Coal], with the addition of the Three Quarter seam, which is met at the depth of 54 feet above the Ballarat, the total depth of the shaft being 25 fathoms. There are about 350 men and boys employed in coal getting and coke making when the pit is in working order."

Whellan's 1894 Directory of County Durham via Durham Mining Museum

NEHL - The pit was substantial in size, with 2 long rows of coke ovens, multiple slag heaps connected to a small tramway to transport the waste. The colliery was directly connected to the Sunnside Branch of the NER.