The Stacks at Downpatrick Head date back 350 million years are made of limestones and shales.
The thin dark layers are shale formed by mud deposits, the medium layers are sandstones, while the white thick layers are limestones and made of seashells. Dun Bristle, the sea stack off Downpatrick Head, is a good example of this feature.
The high cliffs were formed by the drift of the American continent against Europe as well the similar cliffs at Slieve League in Donegal.Dún Briste (Gaelic for Broken Fort) was once joined to the mainland.
The sea stack stands 45 metres (150 feet) tall. Dun Briste and the surrounding cliffs were formed around 350 million years ago (during the ‘Lower Carboniferous Period’), when sea temperatures were much higher and the coastline at a greater distance away.