The Mourne Mountains contain twelve peaks over six hundred metres in height and include Slieve Donard, Northern Ireland’s highest mountain at 850m. To the north the Slieve Croob Massif stands out as distinct rocky summits with thin grass cover and shattered rocky screes. The land rises to a height of 534m at Slieve Croob.

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. 

The high Mournes granites developed a mere 56 million years ago, 10 million years after the dinosaurs had become extinct. During this time there was a huge amount of volcanic activity, as the great continents moved apart leaving what is now the North East Atlantic Ocean. This was also the time when other famous geological features formed in Northern Ireland, including the Giants Causeway and the Ring of Gullion.

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.

Once formed, however, it took millions of years and at least six ice ages scouring the earth to reveal the Mourne mountains. The Silurian rocks probably originally formed a complete ‘roof’ over the granite intrusions - but over millennia the softer rock was gradually eroded away by weather. Only a tongue of Silurian rock penetrating into the heart of the high Mournes at the Deer’s meadow, in which Spelga reservoir now lies, remains of that ancient ‘roof’.

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.

The country to the north and east of the Mournes was covered by material moved and shaped at the base of the ice sheets as they advanced and retreated, and formed into the classic drumlin ‘basket of eggs’, of low rounded hills and poorly drained hollows.

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

Along the coast, moraines, piles of drift material pushed by the edge of the advancing glaciers, were deposited when the ice retreated. Today these form soft cliffs where the sea has cut into the glacial till, exposing a wave cut platform. This is often backed by a narrow band of raised beaches of cobble or shingle formed when the sea levels were higher some 6000 years ago. In the vicinity of Kilkeel these soft cliffs of drift material show shoreline retreat rates that are the highest in Northern Ireland.

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.