Landscape & Heritage
The landscape we see today is not just a natural wonder but has been moulded by centuries of human activity.
The Landscape
The landscape we see today is not just a natural wonder but has been moulded by centuries of human activity. Nature has combined with man’s influence to produce a living, working landscape with special traditions.
The area is renowned for its many dry stone walls that help to divide the land into a “patchwork quilt” of fields. The most famous of these walls is the magnificent Mourne Wall, which celebrated its centenary in 2022. Owned by NI Water, it was designated as a listed structure in 1966. In recent years, the wall has undergone significant repairs. The Trust is represented on the working group that oversees this important work.
The Trust’s Ranger and Countryside Teams maintain access to important heritage sites such as Legananny Dolmen and the Granite Trail at Newcastle Harbour. They also monitor and carry out repairs to features such as dry stone walls. Their knowledge and skills are often shared with volunteers and landowners so that they too can make a positive contribution to the landscape.
Geology & Landscape
Have you ever wondered why the Mourne Mountains and Slieve Croob rise over a beautiful landscape? The answer lies deep in the prehistory of the earth, beginning when the surface of the earth looked very different. If you stand in the Deer’s Meadow near Spelga Dam, you are standing on the sedimentary rock formed in the earth’s ancient oceans that once covered the Mourne peaks. It took the shifting of continents, volcanic activity and the effects of the various Ice Ages to shape what we now appreciate as the Mourne AONB. This transformation took millions of years to complete.

Ancient Rock
The Mourne Mountains contain twelve peaks over six hundred metres in height. Slieve Donard, Northern Ireland’s highest mountain, is the tallest at 850m. To the north the Slieve Croob Massif rises to a height of 534m.
While granite mountains dominate the Mournes, much of the area, like most of Co. Down, is underlain by Silurian rocks – 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. When this ocean spread, Scotland and Northern Ireland were separated from England, Wales and Southern Ireland. The world really did look very different then – unrecognisable from what we have today.

Volcanic Activity
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. The result was the great continents moving 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.
Newry’s Granodiorite complex, however, is much older than the high Mourne granites. This complex crystallised in the Caledonian period – about 400 million years ago – when the ancient lapetus Ocean closed. These granites are seen in the summits of Slieve Croob, Slievenisky and much of the hilly landscape north and west of Castlewellan.
The mountains formed out of molten magma, but they were not volcanoes. The rock came from the earth’s centre but never quite broke through the Earth’s surface as lava. It bubbled up inside the Earth’s crust. It then 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 & Water
Once formed, however, it took millions of years and at least six ice ages to reveal the Mourne mountains. The Silurian rocks probably 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 remains of that ancient ‘roof’. This is located in the heart of the high Mournes, at the Deer’s meadow, at Spelga reservoir.
Once the underlying hard granite revealed itself to the elements, the great ice sheets 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 moved from Scotland or the north and west of Ireland. These were deflected around the mountains creating the ‘fjord’ of Carlingford Lough, and the adjacent Mourne plain complex.
