Clay is a fine grained sedimentary rock. There are clays of different ages in Northern Ireland. One clay of Jurassic age is Lias Clay. This clay is grey in colour and when wet becomes very soft. It underlies the Cretaceous Ulster White Limestone and, especially around the Antrim Coast Road, can cause landslips. These clays are very rich in fossils ranging from crinoids (a type of starfish fastened to the seabed by a stem) to ammonites and Gryphaea (an ancient oyster-like bivalve).
Other much younger clays are found close to Lough Neagh (see map). These are Tertiary in age forming after the volcanic eruptions. The earth movements associated with the period led to faulting and warping of the earth's crust. A basin around Lough Neagh was formed and erosion of the surrounding land led to the formation of clays of almost 350 metres in thickness in some places. Also associated with these clays are layers of lignite which have not yet been commercially exploited.
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