Summary of site:
In early Carboniferous times the sea in the south progressively invaded a northerly landmass. The exposure on the extreme north western shore of Keenaghan Lough shows something of the nature of the onset of this marine incursion as well as revealing its probable date and something of the conditions of the time.
The earliest Carboniferous rocks of this area belong to the Keenaghan Shale Formation and at this locality can be seen to rest on the oldest rocks known in Northern Ireland, the Lough Derg Group. The Keenaghan Shale Formation consists of about 20 m of predominantly black shales, some silty, with thin, lens-like beds of lime-rich grey sandstone. They are tilted 20 degrees to the south here and one finely laminated limestone is seamed with desiccation cracks filled with dark shale.
A temporary outcrop in black and silty shales, 450 m east of the shore locality yielded plant spores of 9 species which give a tentative age in the early part of the Chadian stage, not quite 350 million years ago.
The earliest advances of marine conditions in this area encroached onto a land surface of ancient metamorphic rocks. The environment at the time was hostile, very hot with high evaporation rates, creating high salinities expressed as salt crusted shoreline that baked and cracked (the desiccation cracks). This association of rocks is called the sabkha facies, similar to the shores of the Red Sea and the Persian Gulf today. These conditions were not universal at the time because rocks of similar age within the immediate region are quite different.
The time of these events is not conclusively proved to be early Chadian but, by local comparisons, it seems likely. The area was then on the edge of a supercontinent riding deep convection currents at imperceptible speeds as it approached the equator from the south.