700-540 Million Years Ago
The Birth of an Ocean – Neoproterozoic Era
Our geological story starts 700 million years ago in the Neoproterozoic Era, the final part of the Precambrian. At this time, there was one supercontinent in the Southern Hemisphere that we now call Rhodinia. The only parts of Ireland that were part of this continent were the island of Inishtrahull off of Co Donegal, part of the Mullet Peninsula in Co Mayo and the area south of Bray, including the Sugarloaf, in Co Wicklow.
The supercontinent Rhodinia started to rift apart (i.e. fissures slowly opened up and divided the continent) and a new ocean called the Iapetus started to open around 590 million years ago. As the two continental blocks continued to move apart, magma erupted to form a new ocean floor and sediments such as mud, sand, and gravel were deposited and limestone formed.
We see rocks of this age in Ireland in both the southeast in Co Wicklow and Co Wexford and in the northwest in Connemara, north Co Mayo, Co Sligo, Co Tyrone, Co Donegal and Co Antrim. The rocks to the southeast are associated with a vast continent we call Gondwana and the rocks to the northwest are associated with a continent we call Laurentia.
Laurentia includes parts of Scotland, Greenland, and Eastern Canada whereas Gondwana includes parts of southern Britain, the eastern edge of North America and the whole of Africa, South America and Antartica.
In the Joyce Country & Western Lakes geopark region, we see evidence of rocks of this age from Corr na Móna westwards and from Cur Hill in the Maam valley to south of Maam Cross. Our internationally famous rock, Connemara Marble, started off as limestone at the centre of the new ocean floor. The soft Lakes Marble Formation of the Maam valley was once limestone and sand found in the shallow waters of the young Iapetus Ocean, and the tops of the Maamturks and the Twelve Bens were once sand from the continent deposited on this ocean floor.
490-360 Million Years Ago
The End of an Ocean – Ordovician to Devonian Periods
Devonian Galway granite
As the Iapetus Ocean finally closed at the end of the Silurian, the two parts of Ireland that originally formed in different continents became joined as one. During a time period called the Devonian (420 to 360 million years ago), a large amount of melting occurred locally which resulted in a vast intrusion of magma (dated at about 400 million years ago) that cooled beneath the surface and formed the Galway Granite, which is found all over southern Connemara. It is visible in contact with the older magmatic arc rocks of Connemara at Glentrasna road, south of Maam Cross.
The mountain building process that started in the Ordovician around 460 million years ago created a considerable mountain range. However over the course of 100 million years, erosional forces wore the mountain range down, as evidenced today by the plateau of Maumtrasna. In fact, the metamorphic rocks of Connemara and the Dalradian rocks only represent the roots of this mountain range that has since disappeared.
355-300 Million Years Ago
Connacht in the Tropics – Carboniferous period
300-66 Million Years Ago
The Last 300 Million Years – After the Carboniferous
Cracks in the bedrock were formed and were “plugged” by limited intrusions of magma in our geopark area, as seen by the dolerite sill on top of the Partry mountains at Droimchogaidh. This rifting activity that separated Ireland from Greenland and North America is also now thought to be responsible for the formation of the mountains that we see today in the west of Ireland. A section of Carboniferous sandstone that predates the Carboniferous limestone of Ireland is found on top of the Maumtrasna plateau, which sits 400 m above the sea level. The same sandstone is also found at a depth of 300 m below the surface around Clonbur. This indicates that the rocks west of Lough Mask and Lough Corrib were uplifted by about 700 m. This would have occurred when the early spread of oceanic crust pushed the two landmasses apart, which would have locally pinned blocks of bedrock against one another in a limited space and pushed them upwards.
66 Million Years Ago To Today
Ireland Under The Ice
These cycles have settled to be about 100,000 years long with a short interglacial period compared to the glacial one.
We have very limited information of the penultimate glaciation in Ireland, but we do have extensive information of the last one, which was most extensive during the Last Glacial Maximum, about 24,000 to 21,000 years ago. Extensive amounts of sediments were deposited under the ice sheets and can be found throughout Ireland. These sediments are known as glacial till and are composed of sediments of different sizes, from mud and silt to sand or even boulders. These large fragments of rocks can be attributed to their region of origin and have allowed geologists to trace the movement of the ice. Sedimentary ridges of sand and gravel marking the edge and extent of the ice sheets were found on the marine shelf offshore, up to 150 km away from modern Irish shores. There is evidence of extensive weathering of the bedrock of Irish mountains with only a few areas around some of the peaks with loose sediments left in situ (known as block fields). All other loose sediments were carried away by the moving ice. This collection of evidence shows that at its maximum, the ice covered the whole of Ireland, most of the mountains (leaving only small peaks outcropping over the top of the ice) and extended up to 150 km offshore. This last ice sheet eroded the bedrock, shaped the landscape and carried away an enormous amount of sediments offshore.