The southern part of Donegal has had a long and complex geological history. The oldest rocks in the area of this map are the quartzites that form Carnaween in the west of the area. These rocks are from the Proterozoic era and are over 600 million years old. The other major rock types from this era are the Lough Mourne and Lough Eske Formations. These were sedimentary rocks and have been subjected to heat and pressure which has converted them to metamorphic rocks.

The Barnesmore Granite was intruded into the Lough Mourne and Lough Eske Formations 400 million years ago during the Devonian period. The granite was molten when it ascended through the earth’s crust and solidified in its present location. The granite is made up of three different types of granite and forms the hills from Barnesmore in the east to the Struell Valley in the west.

Then 300 million years ago in the Carboniferous period, sandstones, limestones and shales were laid down under water. At this time there was a precursor to Donegal Bay which was larger than the present day bay. It stretched from Killybegs in the west through Lough Eske in the centre to Laghy in the east. The Carboniferous rivers were eroding the same hills as we see today (although these would have been much higher and less rounded) and carrying the sediment into the bay to form the sandstones which occur around Drumkeelan and upon which the stonemasonry industry is based.

Sometimes the Carboniferous rocks lie unconformably on top of the Proterozoic rocks and sometimes they have been faulted against them by earth movements. One of these faults is the Boundary Fault which trends from Killybegs to Lough Eske, and exerts a fundamental control on the landscape in this area. The higher ground lies to the north of the fault and comprises Proterozoic metamorphic rocks and the Devonian granite at Barnesmore. The ground is higher because these rocks are harder and more resistant to erosion than the softer Carboniferous rocks that lie to the south of the Boundary Fault. The fault is drawn as the change in slope at the base of the mountains.

However, most faults in the area trend northeast-southwest and control many of the features and landforms in the area. The Leennan Fault, which lies to the west of Carnaween, continues southwestwards into Mayo and northeastwards into Scotland. It is part of this fault system that created the weakness in the rocks that allowed the sea to erode the natural harbour at Killybegs. Other faults that transect the area are the Carnaween, Belshade, Barnesmore Lough and the Laghy Faults. One of the most visible faults is the Barnesmore Fault which is the control on the landscape in the Barnesmore Gap. All methods of communication and services follow the road through the gap (road, rail, power, telephone).

Then, some 60 million years ago during the Tertiary period, basalt dykes (long narrow linear features) were intruded. These generally trend northwest-southeast and can seen on some mountains as ridges or depressions, depending on how they are affected by erosion. There are also several basalt plugs (circular, hill-size intrusions) around Donegal Town. These rocks are the same age and type as those that formed the Giants Causeway in Co Antrim.

One of the most significant events to have affected Donegal was the glaciation of the Great Ice Age. This ended some 13,000 years ago, comparatively recently in geological time, and is responsible for sculpting the landscape that we see today. Southern Donegal has well developed and well preserved examples of upland and lowland glacial features.

Glaciers would have been present in the Struell, Eglish and Barnesmore areas and at times most of the area would have been covered by an ice sheet, hundreds of metres thick. The glaciers exploited the already existing river systems and changed the shape of the valleys from V-shaped to U-shaped. This resulted in the development of upland features such as hanging valleys. The ice sheet and glaciers carried the material eroded from the uplands to the lowlands where it was deposited whenever temperatures rose and the ice melted. Features characteristic of glacial deposition are drumlins, kettle hole lakes and kames.

Associated with the temperature rise and melting of the ice sheets was a rise in sea level. This is seen spectacularly in Donegal Bay where most of the islands in the bay are actually ‘drowned drumlins’.