Geology of Ireland

The basalt columns of the Giant's Causeway together with geologically significant sections of the adjacent coast have been declared a World Heritage Site.

[5] From the evidence of the Oldhamia fossils found at Bray Head in Leinster, both parts of Ireland were below sea level at this time.

Fossils discovered near Clogherhead, County Louth, show the coming together of shoreline fauna from both sides of the original dividing ocean.

The Irish landmass was now above sea level and lying near the equator, and fossil traces of land-based life forms survive from this period.

These include fossilised trees from Kiltorcan, County Kilkenny, widespread bony fish and freshwater mussel fossils and the footprints of a four-footed amphibian preserved in slate on Valentia Island in Munster.

The limestone that was exposed by the disappearance of its sandstone mantle was subject to solution by weakly acidic water resulting in a karstic landscape that can still be seen in the Burren in County Clare.

Then, about 150 million years ago, Ireland was again submerged, this time in a chalky sea that resulted in the formation of chalk over large parts of the surface.

As the climate cooled, soil formation slowed, and a flora and fauna that would millions of years later be familiar to the first human inhabitants began to emerge.

[15] Since about 1.7 million years ago, the Earth has been subjected to a cycle of warm and cold stages and these have, inevitably, affected Ireland.

The earliest evidence we have for this effect comes from the period known as the Ballylinian Warm Stage, some half a million years ago.

The greatest of these, the Esker Riada, divides the northern and southern halves of the island and its ridge once served as the main highway connecting the east and west coasts.

The large central lowland is of limestone covered with glacial deposits of clay and sand, with widespread bogs and lakes.

An area of particular note is the Giant's Causeway on the north coast, a mainly basalt formation caused by volcanic activity between 50 and 60 million years ago.