Wicklow Mountains - Geology

Geology

The Wicklow Mountains are primarily composed of granite surrounded by an envelope of mica-schist and much older rocks such as quartzite. The oldest rocks are the quartzites of the Bray Group that include Bray Head and the Little Sugar Loaf and Great Sugar Loaf mountains. These metamorphosed from sandstone deposited in the deep waters of the primeval Iapetus Ocean during the Cambrian period (542-488 million years ago). Layers of sediment continued to form slates and shales along the ocean floor mixed with volcanic rock pushed up as Iapetus began to shrink by the process of subduction during the Ordovician period (488-443 million years ago). These rocks now underlie the uplifted peneplain of the Vartry Plateau between the Bray Group and the main range.

Iapetus closed up completely at the end of the Silurian period (443-415 million years ago) and the Wicklow Mountains were uplifted during the main phase of the Caledonian orogeny at the start of the Devonian period (415-358 million years ago) when the continents of Baltica and Laurentia collided. The collision pushed up a large batholith of granite, known as the Leinster Chain: this is the largest continuous area of granite in Ireland and Britain and runs from the coast at Dun Laoghaire in County Dublin to New Ross in County Wexford and includes the Wicklow and Blackstairs Mountains. The heat generated by the collision metamorphosed the slates and shales surrounding the granite into schists which formed an aureole (shell) around the granite. The process of erosion has removed much of the surrounding schist from the mountain tops, exposing the underlying granite. Some remnants of the schist roof remain on some of the mountain tops, most notably Lugnaquilla. The round granite topped peaks contrast with the sharper schist peaks: for example, War Hill (granite) and Djouce (schist).

The last major geological event to shape the Wicklow Mountains was the Quaternary glaciation during the Pleistocene epoch (2.6 million to 11,700 years ago). The ice deepened and moulded the valleys into the U-shape that characterises the Wicklow Glens, such as Glendalough and Glenmacnass. As the ice melted, small glaciers were left in corries where moraines now dam lakes such as at Loughs Bray and Nahanagan. Corries without lakes also occur, such as the North Prison and South Prison of Lugnaquilla. Escaping meltwater cut narrow rocky gorges at several locations including the Glen of the Downs, the Devil’s Glen and The Scalp. Ribbon lakes, such as Lough Dan and the lakes of Glendalough, also formed.