Geology of Donegal

• Donegal’s rugged scenery is the product of a long, and at times turbulent, geological history.
• As you travel in a westerly direction across the county, you cross several ranges of mountains, running in parallel lines with a very clear south-west to north-east trend. The high mountains are made of crystalline quartzite, and of granite that welled up as molten rock from deep within the earth’s crust. These rocks tell the story of a continental collision and the Caledonian mountain building event that took place over 400 million years ago. Their trend faithfully reproduces the direction of the original collision, as seen in the rugged range of quartzite mountains that stretches north-eastwards from Errigal to Muckish.

• The origin of the quartzites and schists that are now exposed in much of Donegal lies in an ancient ocean that opened and closed over many millions of years. The quartzites started out as sands that were deposited in shallow waters, and the schists as deeper water muds and silts. The sea bed where these sediments accumulated lay close to what is now the eastern coast on North America, near to Greenland, Labrador and Rockall; magnetic evidence indicates that the area was then located in high southern latitudes, near the Antarctic circle.
• The continuous slow movement of the earth’s tectonic plates steadily carried the ancestral building blocks of Donegal northwards, and the continental collision that caused the mountain building occurred as the original ocean closed. Once its crust was consumed in the depths of the earth, the continents themselves collided, just as India is today colliding with Asia to form the Himalayas.
• The sands and muds were buried and baked many kilometres below the ancient mountains. This process altered the rocks from their original form, resulting in the formation of quartzite and schist.
• A further important event in the Caledonian mountain building event was the intrusion of masses of molten granite, probably in areas of weakness as the crust relaxed after its initial compression. There are numerous granite exposures in the county, the most spectacular being the ‘Main Donegal Granite’ which forms the rugged grandeur of Glenveagh National Park.

• The continental collision briefly described here was the first in a series of events that led to the formation of the supercontinent Pangaea. Ireland lay in central Pangaea, and there is much evidence of the Pangaean story in other parts of the country. A few sandstone outcrops in the north and south of the county tell a little of the early history of this period, but most later rocks have been stripped away from the Donegal surface by erosion.
• The mountain building process was accompanied by the formation of numerous faults, which follow the south-west - north-east trend. Perhaps the most remarkable of these is the Gweebarra fault. It runs in a straight line (very unusual in nature!) from near Mulroy Bay, defines the unerring course of Glenveagh, and then provides a convenient south-westerly route to the sea across South Donegal for the Gweebarra river.

• The North Atlantic Ocean opened quite late in the break-up of the pieces of Pangaea, from about 65 million years ago. The crust was stretched and ruptured, and molten rock was driven to the surface; this is now seen in Donegal as dolerite dykes that cut through older rock. The actual continental split occurred to the west of Donegal, so a section of crust that originated as a part of North America is now firmly attached to Europe.
• In Geological terms, the last Ice Age is very recent. The final glaciers to cover the county retreated little more than 10,000 years ago, not long before early humans came to Ireland. The ice left its characteristic marks, scraping the valleys into U-shaped troughs, cutting new paths to the sea through the mountains (such as the Muckish Gap and Glen Swilly), and leaving behind a residue of clay holding a chaotic mass of boulders and pebbles.
• Rocks with a turbulent history such as Donegal’s typically contain metal deposits that have been carried to the surface areas. Deposits of ores including uranium, lead, silver and aluminium have been found, but their concentrations – perhaps fortunately! – are too low for economic mining. Gold has been found in streams and rocks in Donegal; the Geological Survey of Ireland is reticent about the exact locations of these finds. There are also volcanic vents of the type that can bring diamonds to the surface; but no occurrences have been published.

• The ancient rocks that are believed to underlie the North of Ireland are exposed in just one place in Donegal, the wild, uninhabited island of Inishtrahull North-east of Malin Head. These gneisses have been radiometrically dated to an origin about 1.9 billion years ago. Their story is an important link in understanding the geological connections between Ireland, Scotland, North America and Scandinavia. A visit to Inishtrahull is well worth the effort, and includes the rare experience of walking on Ireland’s foundation stones.

References:
1. Geology of North Donegal, Geological Survey of Ireland, ISBN 1-899702-14-8 (quite advanced, but excellent maps and summaries)
2. The Geology of Ireland, edited by Charles Hepworth Holland, Dunedin Academic Press, ISBN 1-903765-07-02 (again advanced; for the expert reader)
3. Geological History of Britain and Ireland, edited by Nigel Woodcock and Rob Strachan, Blackwell Science, ISBN 0-632-03656-7 (designed for undergraduates, but very readable, with excellent explanations of geological processes)
4. Teach Yourself Geology, David A. Rothery, Hodder Education, ISBN 0-340-86753-1 (a good first stop for amateur enthusiasts).
5. Dictionary of Geology, Philip Kearey, Penguin Books, ISBN 0-140-51494-5 (this will help with the heavier tomes).