The earliest rocks to be seen near Kilclooney began as sand silt and mud laid down in a shallow sea over 700 million years ago.These became buried beneath great depths of younger sediments. Extreme temperature and pressure gradually converted the oldest sediments into very hard metamorphic rocks.



About 400 million years ago a meteorite measuring about a kilometer across crashed to earth and penetrated the solid crust close to the Dolmen Centre. Melted by the heat of the liquid mantle, crustal material rose into the resulting crater to form a granite pluton 10 kilometers in diameter.

Surrounding this pluton, cracks in the crust were invaded by explosion breccias and smaller plugs of mantle rock (that crystallised as it rose from the earth's interior).

Massive uplift, beginning about 65 million years ago, resulted in deep erosion. This removed about 10 kilometers of overlying softer rocks and sliced right down to the rugged old rocks of Donegal. It revealed the deep roots of the meteorite crater, opening for study a feature that is unique in Ireland.

The final chapter in the geology story tells about the great ice-sheet that covered Kilclooney. It melted only 15,000 years ago, and left behind the U-shaped valleys, corries, rock-basin lakes, drumlins and erratic rocks that can be seen all around the Dolmen centre.