Our Geological Story

The Mourne Gullion Strangford Geopark is unique amongst UNESCO Global Geoparks as it tells a tale of two oceans through just over 400 million years of geological history. It charts the closure of the Iapetus Ocean and the bringing together of the two parts of what we now know as the island of Ireland, and the birth of the North Atlantic Ocean. The Iapetus Ocean first formed around 700 million years ago and lay between the continent of Laurentia (modern day North America, Greenland, Norway, Scotland and the northern half of the island of Ireland) to the northwest, and the continents of Gondwana (modern day Africa and Australia) and Avalonia (parts of modern-day England, Wales, and the southern half of Ireland) to the south.

The ocean began to close around 450 million years ago during the Ordovician period, and by the Silurian period, northward subduction was occurring bringing the the two parts of the island of Ireland together. Evidence for this activity is in the form of Silurian and Ordovician meta-sedimentary rocks that were formed on the floor of the Iapetus Ocean, with the layers of near-vertical sandstone and mudstone often tightly folded. This sequence of rocks formed as sediment was scraped off a northward facing subduction zone.

At final closure of the Iapetus Ocean around 400 million years ago, and the coming together of the two parts of the island of Ireland, crustal melting occurred and large igneous bodies were emplaced along the line of ocean closure. In the Mourne Gullion Strangford Geopark, three adjoining bodies intruded from northeast to southwest in the early Devonian to make the Newry granodiorite and today extend from Slieve Croob in the northeast to Forkhill in the southwest.

The opening of the North Atlantic Ocean is marked in the Mourne Gullion Strangford Geopark region by two discrete igneous complexes that formed in the early part of the Palaeogene period. The complexes are part of the British and Irish Igneous Province, which is itself part of the North Atlantic Igneous Province. They all have contacts with the Silurian/Devonian end Iapetus lithologies making it possible to tell the story of the closing of the Iapetus Ocean and the opening of the Atlantic Ocean in the same place.

The Slieve Gullion ring-dyke complex dominates the cultural and historical geography of south County Armagh. The complex itself is made up of Slieve Gullion, together with the Ring of Gullion, the name given to the ring of low-lying hills that surround it. Slieve Gullion Forest Park offers the perfect opportunity to explore the entire complex and experience a landscape that has been shaped by fire and ice over the past 400 million years.
The Slieve Gullion area exposes the roots of a complex and violent volcanic system. It consists of three main parts. The first is a ring of around 15km in diameter made up of two main types of igneous (molten) rock – felsite and granophyre. The second is Slieve Gullion, composed of sub-horizontal layers of granophyre, and dolerites and gabbros. The third component is an area of fine-grained granite centred on the summit of Black Mountain in Co. Louth.

Around 58 million years ago a huge mass of molten rock occupied a magma chamber below this area and eventually the stresses generated in the unsupported roof above resulted in collapse. A circular fracture formed around the periphery of the magma chamber, into which magma was injected. This the first phase of eruption formed the felsite that makes up the south west of the ring dyke. This was followed by another phase of eruptive activity occurred forming the granophyre.

The central mass of Slieve Gullion was originally thought to be the core of an ancient volcano, but they are now thought to be horizontal intrusions of magma, that were injected into the preexisting rock. The two different rocks that make up the layers, dolerites / gabbros and granophyres would have originally co-existed in their molten state. However, they become molten at different temperatures so as the magma cooled, two separate rock types formed – granophyre becomes molten at around 800oC whereas dolerite / gabbro becomes molten at around 1100oC.

The last phase of activity was the intrusion of granite centred on Black Mountain, which breaches the ring dyke in the south east, and make sup an area of around 10 sq km. Volcanic activity ceased in the area around 56 million years ago.

At a distance of 10 to 25 kilometres from the ring-dyke features of Gullion, lies the Mourne Mountains Complex consisting of five distinct intrusions as well as minor dyke and conesheet intrusions. The granites were intruded into the Silurian metasedimentary sequences approximately 56 million years ago which have been altered to hornfels close to the contact between the two. Although the granites were intruded at a high level in the crust, there does not appear to have been any volcanic activity.

The earliest phases of igneous activity led to the emplacement of the north-west trending dyke swarm that is well exposed along the coast. This was followed by further fracturing of the crust and the emplacement of elliptical cone-sheets that now encircles the granites.

The main phases of igneous activity however, are the five granite intrusions that are divided into three in the Eastern Mournes Centre, and two in the Western Mournes Centre. These were emplaced at a relatively shallow level with the eastern granites (G1-G3) being emplaced prior to the western granites (G4 and G5). In addition to its geological significance, Mourne granite is notable as a building stone and has been used from Neolithic times to present. This once thriving industry has left its indelible mark on the landscape with many mountain quarries in the area including Thomas’s Mountain Quarry that overlooks Newcastle, as well as the remnants of stonemasons’ shelters built from discarded quarry blocks throughout the mountains.

The granite was used most notably in the construction of the Mourne Wall between 1904 and 1922, made up of a 22-mile-long wall to enclose the catchment area of public drinking water reservoirs. However, Mourne granite has been exported and used around the world in a number of famous locations including the Hans Christian Anderson statue in Central Park, New York, Parliament Buildings at Stormont, Belfast, the “Silence” Water Feature, Connaught Hotel, London and the 911 British Memorial Garden, New York.

The landscape as we know it today was shaped by ice in the last tens of thousands of years with the last glaciation happening between 115,000 and 12,000 years ago. During this time an ice sheet in the highlands of Scotland grew and advanced southwards across to Ireland where it joined with ice sheets from the Irish mountains. Between 27,000 and 23,000 years ago, Ireland was completely covered by an ice sheet up to 1.5km thick, flowing over the top of mountains such as Slieve Donard and Slieve Gullion. The ice scoured the landscape, moving rocks and mud southwards and shaping the landscape into drumlins and ribbed moraines, helping us to understand the direction the ice moved.

The landforms provide evidence of multiple stages of ice development and movement and the ice interaction with the rocks show the stark difference between the harder rocks that make up the high mountains in the region and the relatively softer rocks now found in the lowlands.

The high Mournes have notable evidence of glacial features such as corries, cols, east-west hanging valleys and deep north-south glaciated valleys. Carlingford Lough is a drowned faultcontrolled glaciated valley. The valley floors have deep glacial deposits and surrounding the mountains, the deposits are shaped into drumlins by the ice. The flat areas of the Mourne Plain are covered in a thick fluvioglacial and glaciomarine deposits of muds, sands and gravels including a raised beach feature which allow for good drainage and therefore good agriculture.

While most of the Mourne mountain peaks are ice-smoothed and rounded, Slieve Bearnagh and Slieve Binnian have tors at their peaks. The island of Ireland has only one other mountain with tors however these are the finest examples. They show horizontal and vertical jointing and their existence might be related to postglacial accentuation of fractures or they may have been exposed as nunataks during the last glaciation and therefore resulting in deeper weathering than the rest of the Mournes.

The Strangford Lough coastline is one of the best places to study glaciogenic sediments, particularly from the end of the last glaciation. The stretch of coast at Ballyhornan includes Killard Point, a designated site that records the events and processes that operated in the area from 15,000 to 13,000 years during the closing stages of the final glaciation to affect the island of Ireland. The site is so important that this short period has won a place in the scientific literature as the Killard Point Event.