Northern Ireland - Down-Longford

This area lies within Down and Armagh and shares a border with the Republic of Ireland. The main settlements in this area are Armagh, Newry and Downpatrick. Again, no sedimentary bedrock lies on top of the basement rocks in this area.

Basement rocks

In the Down—Longford area basement rocks are found at the surface and extend to depths greater than 6 km. These rocks are mainly sandstones, with mudstones that range in age from 460 to 430 million years old. The rocks have been slightly altered by temperature and pressure (weakly metamorphosed) and have water in the fractures only at shallow depths, deeper the fractures are tightly closed due to the weight of the overlying rocks. The sandstones are used for aggregate, the best of which is used for road surfacing. Gold has also been found associated with these rocks and may become economically important in future years.

There are two generations of molten rock or magma which were intruded into the basement rocks of this area and then cooled and solidified to form granitic igneous rocks. An older group, found in the vicinity of Newry (from Slieve Gullion to Slieve Croob), are about 420 million years old. The younger group are the granites that form the Mourne Mountains, only 55 million years old, and were formed along with the lavas in Antrim as a consequence of the initial opening of the North Atlantic Ocean. The Mourne granites contain sufficient naturally radioactive minerals to produce geothermal heat. They are exposed at the surface but are not thought to extend to much more than 1 km depth. The older granites extend to several kilometres depth.