Limestone is a very common type of sedimentary rock formed largely of calcium carbonate and mostly deposited in the sea. Some are precipitated directly from water; others are formed from the calcareous remains, often microscopic, of animals or plants. In Northern Ireland most limestones are either Carboniferous in age, found particularly in Armagh and Fermanagh, or Cretaceous, found in Antrim and Derry. The latter is the very pure Ulster White Limestone Formation, about 80 million years old, formed of countless microscopic skeletons of marine algae.

Fossils are common in many limestones. The picture here shows a large echinoid (sea urchin) in the Ulster White Limestone at Whitehead, Co. Antrim.


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