One of the most impressive aspects of Fort Dunree is the landscape in which it is set. The present day form of that landscape is the result of millions of years of volcanic activity and erosion by wind and ice. The Urris hills and the other mountains of North Donegal are the remnants of an ancient mountain range that ran from Norway through Scotland and Ireland into Newfoundland and Canada, before the Atlantic Ocean existed. They are believed to have been higher than the Himalayas are today. The resulting folding and contortions can be seen in the exposed rock of the sea cliffs.
The action of the ice ages, the last ended about 13,000 years ago, helped carve out the sea loughs of Lough Swilly and Lough Foyle surrounding the peninsula of Inishowen. Ice sheets 1,000m thick slowly ground their way along, gouging out valleys and depositing hills of gravel. As the larger ice sheets retreated northward smaller glaciers sculpted the rock. A good example can be seen just over Mamore Gap to the right of the road, where there is a small pool in a corrie. This is the ‘nest’ of one of these smaller glaciers.
Trees gradually returned to the landscape until the climate became wetter about 5,000 years ago and the higher more exposed ground gave way to bogland. We call this type of bogland Atlantic Blanket Bog. The reminants of the ancient woodland can still be seen.