Geology of Donegal

There are many spectacular cliffs in Donegal, this would be due to the different degrees of exposure of wave energy from the Atlantic Ocean, creating platforms of different rock types including rocky shores, storm beaches and the unique sandy mochair grasslands. Many of the cliffs in Donegal, including the Slieve League which reaches 1,950ft (600m) above sea level are made of Pre-Cambrian, quartzite and Gneiss which do not readily form suitable ledges for sea birds, but their height and north- facing exposure makes them the habitat for Arctic and Alpine plants.

Donegal is surrounded by these ancient rocks, which are referred to as the basement rocks of Ireland. Almost all of these rocks have been radically altered from their original state by being subjected to extreme pressure and heat at some stage in their history and referred to metamorphic rocks because of their changed form. The lime- stones have become marbles and mud- stones have turned to hard but brittle slates, sand- stones have formed into the rock type called Quartzite.

Almost all of these altered rocks are more compact and resistant to erosion than their original counter- parts and commonly form uplands or individual mountains. Quartzite and the altered form of sand- stones, are a good example of resistance to the elements, some of Irelands most spectacular mountains are composed of quartzite. Another further distinctive characteristic of these mountains is the bleached appearance of the rocks due to the quartzite, which is composed almost entirely of the grey white mineral called Silica.

The climate of the earth was very cold and Ireland would have looked like Greenland does now, allowing snow and ice to spread over most of the earth. During the times of these cold periods the whole of Ireland was buried beneath ice and snow, which is now is the sea surrounding us. The Irish climate was dry and cold, with intensely cold snowy winters but no permanent cover of snow very similar to the high tundra of Canada and Siberia today. If the snow of the winter before had not melted in time for the next fall of snow by the following year, it would cover that layer and every year the snow would get deeper and deeper, this would compact the snow underneath into ice.

The highest mountains are where this action took place first and thus these mountains developed their own ice caps. When this ice sheet melted it was responsible for the newer deposits of boulder clay over the parts of Ireland covered by the sheet. The valleys were scoured and plucked, widened and deepened due to the constant movement of the glaciers in these valleys, the bared rock surfaces were striated, polished and moulded and the many rock basins, lakes and waterfalls among the mountain valleys were to further indicate the activity of the ice.

Many rivers were forced by glacial deposits to seek new path- ways to the sea; among the mountains, glacial dams held up lakes until the waters escaped in many cases through spillways. When the melted water from the ice raised the sea level above the low-lying coasts so that the sea poured over the entrances to the lower valleys. Since the ice age, the other agents of weathering and erosion have been active. Wind and water have spread the debris over the plains and along the coasts. The poor drainage and heavy a rainfall, together have produced the solution lakes and the growths of peat which are so typical of our present Irish scenery.

Also from the forces of nature acting on the solid rocks of the country are the various landscape features it produced. Soil itself is the link between the rocks and the landforms and also animal and plant life, it has a passive function, the soil blankets the land smoothing the rough edges in the rocks below and creating the long swelling slopes that are also a characteristic of the Irish scenery.