Summary of site:
2 km north of Gortin there is an area of around 25 sq km of grey-green sandstones with purple-red and grey blocky mudstones standing unconformably (with a time break) and in isolation on the ancient Dalradian basement rocks of the area. Minor constituents are limestones with colonies of calcareous alg'. The best succession is seen in Middletown Burn on the south side of Slievebeg.
These rocks have been allocated to the Omagh Sandstone Group of the Carboniferous period and are generally described as the Newtownstewart Outlier. They are unlike the typical marine Omagh Sandstone Group rocks because they appear to be entirely terrestrial lake deposits and they have a thickness of between 1,200 to 1,500 m, considerably thicker than their marine equivalents. Another curiosity is their lack of a basal conglomerate or similar deposit.
While there are no marine fossils, samples of the grey, blocky mudstone have yielded plant spores of over 20 species which, together, indicate a very early Carboniferous date. They belong to the late Tournaisian, the first epoch of the period, formed around 360 million years ago.
The only explanation for such a thickness of lake sediments so far advanced is that they formed in a land-locked basin subsiding between faults. Structures in the sediments suggest a supply of sediment from the south. Thick sequences of late Tournaisian terrestrial sediments are unknown elsewhere in Ireland either onshore or on the continental shelf but there are almost identical thick sequences of similar age in Nova Scotia on the Canadian eastern seaboard. A reconstruction of the geography of the Tournaisian shows both Canada and Ireland shared the same continent, Laurentia, in a similar adjacent positions inland of the sea. It has been suggested that a more appropriate name for the sandstones would be the name applied to their Canadian equivalents, the Horton Group, but this would mean stretching the conventions of formation naming a little too far.
Since the Middletown Burn section is the best in the outlier it should be offered protection to ensure its continuing availability for teaching and research.