Limestone is an essential mineral for use in the smelting of iron ore to make iron. It is added to the blast furnaces to provide a flux within the chemical changes taking place under great heat, the impurities in the ore combining with the lime to be drawn off the furnace as slag. In modern times hard limestones are much prized for the production of aggregates for use in the road building and construction industries. Lime is also used extensively for the manufacture of cement, and in agriculture for the sweetening of acid soils to make them more productive.
Limestone was an significant originating traffic on the railway over Stainmore, most of this coming from the Hartley Quarry, about 1 mile south east of Kirkby Stephen town.
The quarry was opened in 1927 by Sir Hedworth Williamson Limeworks Ltd. The company had long operated quarries at Fulwell Hills, near Sunderland in County Durham, but these were becoming exhausted and the Hartley site not only had vast reserves of high quality limestone but was on the East-West railway providing new market opportunities in Cumbria and Scotland in addition to maintaining traditional markets in North East England.
The quarry was provided with its own internal railway system which exchanged traffic with the main line railway at Merrygill sidings, just south of Merrygill Viaduct. The site of the sidings can be seen on the level area to the south-east of the main railway formation as the latter descends down the 1 in 72 gradient towards Kirkby Stephen. The ruin of the one-time signal box which controlled the signals and access to the sidings is adjacent to the path at this point.
With the long-term decline of the iron and steel industry, production from the quarry increasingly turned to other markets for which road transport was essential. The railway connection was severed in 1975 involving the final closure of that remaining part of the railway south and east of Warcop through Kirkby Stephen. Demolition of the railway bridge over the road at the north end of Merrygill Viaduct then allowed large road vehicles access to the quarry which continues in operation.