Geologists date the Blue Stack Mountain range as being 470 million years old. Not alone the Blue Stacks, but most of the land mass in the northern half of Ireland. It is believed that at one time this area was the sea bed of a tropical ocean and proof of this is to be found in the Cuilca Mountains in Cavan, where fossilised tropical fish have been located on the mountain top.

The process which resulted in the formation of this area is believed to be the movement and collision of 2 land masses. The resulting volcanic mountains, once higher than the Himalayas, have eroded over the centuries to expose rocks such as Granite, Dollarite and the volcanic Appanite.