Rising some 576m above the surrounding countryside, the broad slopes of Slieve Gullion dominate the landscape of south Armagh. The mountain itself lies at the centre of a pronounced ring of hills – the Ring of Gullion. Together these features are a testament to more violent times in this part of Ireland for they are both of volcanic origin. Both the mountain and its surrounding ring of hills represent the, now much eroded, heart of a volcano that existed here around 60 million years ago. At that time, both Europe and North America were still joined together. However, as the two continents began to move apart (due to forces deep within the Earth), resulting tension in the Earth’s crust caused large volumes of rock at depth to melt and this molten rock, or magma, was erupted at the surface through large volcanoes. By following the Slieve Gullion Forest Park Drive it is possible to see evidence of those more violent times.
Just before you reach the first car park along the drive some of the most interesting rocks on Slieve Gullion can be seen on the right hand side of the road. Here the intricate relationships between the light and dark rocks have caused much debate amongst the scientific community about how these rocks and indeed Slieve Gullion as a whole came to be. It was thought that Slieve Gullion was made up of altered layers of lava that were erupted from the volcanoes here around 60 million years ago. A more favoured explanation however suggests that the layers of dark basalt-like rock and light granitelike rock never erupted as lava at the surface. The dark-coloured rockis called dolerite. Dolerite is an igneous rock that formed from the slow cooling of magma deep in the Earth’s crust. Near-horizontal sheets of dolerites are interbanded the other igneous rock, granophyre, which also formed from the cooling of magma deep underground. Together they make up Slieve Gullion and they give it a somewhat stepped profile.