Summary of site:
The significance of the Ghann River valley deposits to the overall story of events in the western Mournes is covered in the Western Mourne Complex overview (Key Site 450).
In the closing stages of Ireland’s last great glaciation, the Midlandian, the ice that formerly extended from the central Mournes into the adjacent southern lowlands was rapidly melting. The glacial front retreated and eventually decaying mountain glaciers were all that remained of the great ice sheet.
The glacier in the Ghann valley was one of these. Short-term climatic variations meant that mini ice ages periodically checked the glacial retreat up the valleys. When this happened, the snout of the glacier appeared to stand still for a time because its rate of flow was matched by its rate of melting. Muds, sands, gravels and boulders, carried on and in the ice or dragged below it, were released at these times to form heaps—moraines—banked against the glacial snout. These arcuate mounds mark such events and the pronounced feature at the entrance to the Ghann valley (west of Thunders Hill) that curves north-west across the neighbouring Moygannon River is a clear example.
A pair of channels cut in loose debris by meltwater flowing below the ice show as a wandering ridge, up to 10m high, that extends from below the moraine, up the valley, to a point about 500m below Drumreagh Upper. The death throes of the glacier are reflected in the benches of boulder clay that trend north-west/south-east on the western side of the upper valley, cutting north along the steep eastern flank of Slieve Roe. There are broader, equivalent benches on the eastern side of the valley, on the gentler slopes of Slieve Roosley.