Connemara before Clifden: The Geology of Connemara, Ronán Hennessy

photo courtesy of fergal

A journey into Connemara is a journey into a landscape of spectacular scenery and ancient history. The natural backdrop of mountains, valleys, lakes and rivers in Connemara is the result of natural events that occurred over the last 600 million years since Connemara’s rocks first began to form. Much of this history centres on the closure of a great ocean: the Iapetus Ocean. Over tens of millions of years, two continents (Laurentia and Avalonia) that lay on either side of the Iapetus Ocean slowly converged. The movement of the Earth’s plates during the closure resulted in the creation of volcanic islands, folded mountain belts, and the injection of great volumes of magma into the Earth’s crust. Evidence of these events is visible in the rocks throughout Connemara.

The mountains of the Twelve Bens and Maumturks are the roots of a great mountain belt of Himalayan magnitude that formed during the closure of the Iapetus Ocean. The remnants of this mountain belt can be traced from eastern USA, through Greenland, Ireland, Scotland, and to Scandinavia. Over time, Connemara’s mountains have been worn down to the 700m high peaks we see today. These mountains are made of metamorphic marine rocks called the Connemara Dalradian rocks. Clifden is situated on these rocks, as is much of the N59 (and the old MGWR railway line) from Oughterard to Clifden. The 470 million years old rocks of Roundstone Bog and Errisbeg originated as igneous basalts, and were later metamorphosed due to high pressures and temperatures. The granites of Roundstone, Omey, Carna, and the entire northern side of Galway Bay formed around 400 million years ago, again due to the ocean closure event.

This active landscape finally quietened down by Carboniferous times, some 350 million years ago. During this time the fossiliferous limestones of Oughterard and Lough Corrib were laid down in a shallow, warm tropical sea. Finally, during the successive ice ages that occurred in the past 2 million years, the Connemara landscape was moulded and shaped into the physical landscape that greeted the first settlers to arrive in Connemara. The ‘look’ of the land may have changed since the time of the first settlers, with the introduction of fields, bogs, roads, railways and towns. However, just as much of Clifden town would be recognisable to its first inhabitants, the first settlers in Connemara would today recognise the same mountains, valleys, lakes and rivers. With them, and all who will come after us, we all share Connemara’s geological heritage in this spectacular and ancient landscape.