Chalk is a very pure form of limestone. The chalk in Northern Ireland was laid down in the Cretaceous Period which lasted from 135 Million Years ago to 65 million Years ago. Towards the end of the Cretaceous Period all of the British Isles was under water, except for the highest parts of Scotland. This sea had many algae living in the warm waters. As these algae died their skeletons accumulated on the sea bed. The skeletons, or coccoliths, were made of calcite and it is this accumulation that has formed the chalk rock. Because there was so little land nearby, the coccoliths were not contaminated with rock fragments eroded from the land and have produced a very pure form of limestone. Up to 1000 metres of chalk were deposited and, since it took about 30 years for just 1 millimetre to be deposited, it was clearly a very slow process.
The chalk in Northern Ireland is the same age as that in England, for example at Dover. However the Ulster White Limestone, as the chalk in Northern Ireland is known is very much harder than its English counterpart having been affected by the Tertiary lavas which overlie it. The only outcrops of chalk in Northern Ireland are where those Basalts have protected it. Hence the best places to view chalk are along the north and east Antrim coasts.
Flint nodules can be found in much of the chalk. Some of these may have formed from sponges which make their skeletons of silica, but their origin is unclear. Flint was used by Mesolithic peoples who colonised Ireland about 8000 years ago. It has been surmised that the visibility of chalk cliffs from Scotland would have tempted those people over to this part of Ireland.
|Chalk data||Chalk sample||Chalk landscape|
|Flint data||Flint sample||Flint photograph|