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).