Geology of the Causeway Coast AONB

The oldest rocks in the area are restricted to a few places along the coast such as Portrush and White Park Bay. These are of Jurassic age (190-135 million years ago) and consist of sediments laid down in the warm waters of those times. Sub-tropical oceans were still a feature in the succeeding Cretaceous period (135-65 million years ago). The white limestone or chalk was formed from the remains of plants and animals. They contain fossil evidence of the animals which inhabited these ancient seas. By the end of the cretaceous period, relative sea levels had fallen leaving dry land. The brilliant whiteness of the chalk contrasts with the more recent basalts, as does their modes of origin.

Volcanic and fissure eruptions spewing out magma and ash were related to the opening of the modern Atlantic ocean during the Tertiary period (62-2 million years ago). The basalts are mainly 65-60 million years old. The Lower and Upper Basalts represent the more prolonged periods of eruption, with successive flows increasing the thickness of basalts, the resultant rock formed by the cooling lavas. The intervening Middle Basalts were formed in a similar manner but during this period violent volcanic eruptions were more frequent as at Carrick-a-Rede and Kinbane.

Eruptions were not continuous and during quiet periods weathering of the basalt surface occurred producing tropical soils. One outcome of an extended period of such weathering was the major inter-basaltic horizons to be found between the Upper, Middle and Lower Basalts. In places this weathering progressed to the point where enriched iron and aluminium ores formed and these have been commercially exploited in the past. Partially rotted vegetation also accumulated in places forming lignite as at Ballintoy.