How the Mournes were Formed
The underlying geology, subsequently shaped by weather and human activity to give the landscape we know and love today, is an appropriate starting point for explaining the special character of the Mourne area.
This section guides you through the periods and processes that did most to produce our wonderful landscape.
Ancient Rock

While granite mountains dominate the Mournes, much of the area, like most of Co. Down, is underlain by Silurian rocks of shales, mudstones or greywackes. These formed over 420 million years ago, from mud, sands and silts lying at the bottom of an ocean known as the Iapetus Sea, which once separated, on the one hand, Scotland and Northern Ireland from England, Wales and Southern Ireland on the other. The world really did look very different then, the layout of the oceans and continents unrecognisable from what we have today.
Volcanic Activity

Much older than the high Mourne granites, however, is the Newry Granodiorite complex, which crystallised in the Caledonian period - about 400 million years ago - in association with the closure of the ancient Iapetus Sea. These granites are seen in the summits of Slieve Croob and Slievenisky and much of the hilly landscape north and west of Castlewellan.
The mountains formed out of molten magma, rock from the earth’s centre, but they were not volcanoes. The molten rock never quite broke through the Earth's surface as lava, but bubbled up inside the Earth's crust and slowly cooled beneath the overlying sandstone into the interlocking crystals of quartz, feldspar and mica that form granite. The granite mass that is now exposed was formed when blocks of Silurian shale subsided, leaving a cavity which was filled by an up welling of acid magma.
Ice and Water

Once the underlying hard granite revealed itself to the elements, the great ice sheets which occurred over the last 2 million years, further carved and shaped this robust rock into the enduring domed peaks we see today. Intense glacial erosion created steep sided U-shaped valleys, hanging valleys, and other features such as corries and valley moraines. During later glaciations, massive ice sheets moving from Scotland or the north and west of Ireland, were deflected around the mountains creating the ice deepened ‘fjord’ of Carlingford Lough, and the adjacent Mourne plain complex south of the Mourne massif, now a narrow undulating plain of coastal lowlands and extensive mud flats in Mill Bay.
Below the Mountains

The flatter Kilkeel plain on the other hand formed from sands and gravels which came as outwash from the melting glaciers.
Reaching the Sea

Glacial materials deposited on the floor of the sea have also provided abundant sediments, which have been reworked by rising sea levels over the milennia. Geo-morphologically important massive coastal features of shingle and sand have been constructed across Dundrum Bay, forming extensive sand dunes of Murlough and Ballykinler bounded by a wide shallow bay with sand and shingle beaches and mud flats.
