Walk 3: Ice Age
The actual shapes of the mountains and valleys we have today are the result of the Ice Age. A series of cold stages in Earth's climate between 1.6 million and 10,000 years ago caused ice sheets and glaciers to advance and retreat over northern Europe. Glaciers carved the rocks of the uplands and partially covered the lowlands with deposits of boulder clay (a mixture of boulders and clay!), sands and gravels.
The glacially carved Dirks, seen from the roadside at Shanvallycahill
Park on the road side at Shanvallycahill, ~400m west of the bridge over the Owenbrin River [GR 053 625]. Looking to the northwest up the Owenbrin valley we see several fine features of a glacially produced landscape. The main valley has the typical open U-shape produced by a glacier slowly grinding its way forward. The amphitheatres on the left (west) side are corries, where small glaciers feeding into the main valley carved hollows for themselves. The valley bottom has a thick fill of the material ground from the valley and corrie sides, deposited as the glacier retreated by melting. The corrie glaciers left mounds of material across the mouths of their hollows, which dam small lakes in the corrie floors. The river's pebbly bouldery bed results from washing the finer part of these glacial deposits away (into Lough Mask to form the sand banks known to fishermen) and leaving the coarser part behind.
As an epilogue to the geological story, walk down the signed track towards the lake shore, noting the pine stumps in the bog ("bog oak"). Consider that when the ice finally melted away, a landscape of bare rock and glacial deposits was left to be slowly colonized by plants and animals. We must remember that humans were among these early colonizers, moving up from the warmer lands to the south. As we see from the five thousand year old Ceide Fields in north Mayo, humans watched as the bogs slowly spread over their fields and choked and fossilized the forests. The Ice Age and subsequent landscape evolution are covered in detail in Mitchell & Ryan's book referred to above.