The shales and sandstones are Triassic and Carboniferous in age. That is, they were deposited between 200 and 300 Million years ago.
Some were deposited at the start of the Carboniferous period, before the limestones started to dominate. In Dungannon, for example,mudstones and shales were laid down, interspersed with a type of clay called fireclay. These sources are still exploited commercially in Coalisland. South of the Clougher Valley the carboniferous limestones have sandstone deposited on top with beds of shale contained in the sandstones.
|Shale sample||Fireclays in use|