The Geology of Co Antrim

About 600 million years ago, Ireland lay under the ocean somewhere in the southern hemisphere

About 600 million years ago, Ireland lay under the ocean somewhere in the southern hemisphere. About 510 million years ago, land began to form due to crustal movement, and the part of the earth’s crust where Ireland was to form migrated northwards towards its present location. The island had a recognisable landmass about 340 million years ago, but after tens of millions of years, the land was worn away and largely covered by sea again. Mud rich in the remains of sea-life was deposited on the floor of this sea and gradually formed carboniferous limestone, which covers a large section of Ireland today. With further crustal movement, the limestone cover was thrust upwards approximately 300 million years ago.

About 290 million years ago, the sea invaded large parts of the island leaving deposits of salt. This continued into the Jurassic period, about 190 million years ago, and the clays and schists underlying the basalt and chalk of the Antrim region were laid down. About 65 million years ago, during the Cretaceous age, much of the land was inundated by rising sea levels, and the remains of sea creatures were deposited as soft chalk. This chalk is particularly rich in flint nodules, which are the fossilised remains of Cretaceous sponges. The flint was invaluable to Neolithic people who worked it into tools and weapons. The climate was probably tropical or sub-tropical.

Lava and Basalt
The soft chalk deposits have disappeared from most of Ireland over the millennia, but they have survived in Co Antrim due to being covered by lava during two distinct periods between 56 and 62 million years ago. A large lake of molten basalt about 100m deep formed in what is now north Antrim, and its slow cooling, perhaps underwater, gave time for the particles in the rock to arrange themselves in the unique patterns found at the Giant’s Causeway.

In between lava flows, there was time for weathering of the basalt to occur, and in the tropical climate of the time, red laterite formed. In some places, this laterite was baked by subsequent lava flows to form a very hard, dense rock called porcellanite. This material was used in the late Stone Age to make axe-heads, as at Tievebulliagh, near Cushendall, Co Antrim.

The remains of the lavas are known as basalt, visible everywhere from the River Bann, eastwards to the Antrim coast, and southwards to the Lagan valley. The Belfast Hills, including Cave Hill, mark the southeastern edge of the lava flows. Cave Hill is a basalt layer, which lies over a much older limestone base. This can most easily be seen along the face of the old limestone quarry on the slope of Cave Hill, above Carr’s Glen. The quarry excavation has cut down through the basalt and limestone and exposes the layers clearly.

The phenomenon of basalt overlying limestone can also be seen along the Antrim coast road. This is no coincidence, as the Cave Hill was formed at the same time and by the same volcanic events that created the whole of the Antrim Plateau.

Ice Ages
The huge ice sheets that covered large parts of Ireland at intervals over the last two million years caused the scraping and weathering of the rock formations. These ice ages had little effect in Co Antrim other than reducing the thickness of the basalt. But they had a profound effect on the development of the fauna and flora of the region. In some places the upwelling lava did not reach the surface and cooled more slowly to form a denser rock called dolerite. This resisted subsequent weathering and survives in features such as Slemish, an inclined volcanic plug.

Further Reading:
Nature in Ireland. A Scientific and Cultural History (1997) edited by JW Foster; Regional Geology of Northern Ireland (1972) by HE Wilson.

© Cormac Hamill. Reproduced with kind permission of the Cave Hill Conservation Campaign 2002.