The Tertiary granite intrusions form the Mourne Mountains of County Down and are exposed over an area of approximately 150 square kilometres.
The granite from the mountains has been used from Neolithic times to the present day and was a very popular building stone throughout County Down. The granite was used most notably to build the Mourne Wall- a traditional dry stone wall built between 1904 and 1922 by the Belfast Water Commissioners to enclose the catchment in the mountains for reservoirs to supply the city with water.
The Mourne Wall is 22 miles long and averages 1.5 m high and 0.9 m wide. The wall is now a popular walking route and passes over 15 mountains. There were many mountain quarries in the area including Bloody Bridge and Thomas’s Mountain Quarry overlooking the coastal town of Newcastle. The remnants of stone mason shelters built from discarded quarry blocks can also be seen throughout the mountains.
Mourne Granite is grey to pink in colour, medium-grained with visible quartz, feldspar and clots of biotite with some coarse white feldspar.
The granite weathers by chemical degradation of feldspars and biotite mica and physical disaggregation of individual grains. Biotite mica weathers to a rust brown colour and this often results in discoloration of the surfaces of building stones.