The Ice Age – Glaciers Shape the Landscape
Following the Devonian, there is a gap of nearly 400 million years in the geological record of Glen Roy. By the end of the Neogene, about 2.4 million years ago, the Devonian sandstones and the older metamorphic rocks and granites had been reduced by the action of the sea, and particularly by weathering, landslipping and erosion by rivers, to form rolling countryside not unlike that at present, with some rivers flowing northeastwards and others crossing the line of the present Great Glen in an easterly direction.
About 2.4 million years ago, there was a general cooling of the climate, and another agent in the sculpting of the landscape appeared in the form of glacier ice. At first, the periods of extreme cold were comparatively short, perhaps only 40,000 years or so, and the glaciers were probably confined to the mountains. However, over the last 450,000 years there have been at least four major, intensely cold episodes, during which Scotland was covered by an ice sheet for long periods.
The glaciers and ice sheets removed all the pre-glacial soil and other soft superficial deposits, and excavated steep-sided, flat-bottomed glens, of which Glen Roy and the Great Glen are good examples. On a smaller scale, the erosive power of the carpet of rock debris transported at the base of the ice is apparent from the rocky outcrops that have been smoothed off to form the scratched (striated) and polished rock surfaces of 'whalebacks' and ‘roches moutonnées’; the action was like that of a very coarse sandpaper. Some particularly good examples can be seen in Glen Nevis.
In geological terms this brings us almost up to the present day. We are still technically living in the Ice Age (major ice sheets remain over Greenland and Antarctica), but in a warmer, so-called 'interglacial' interval. The last time an ice sheet covered the whole of Scotland was less than 20,000 years ago during the Late Devensian glaciation. Following its disappearance during a warmer interval, glaciers again entered the Glen Roy area during the brief, but very cold period of the Loch Lomond Stadial, which is thought to have begun about 12,900 years ago. The glaciers may have persisted until about 11,500 years ago, a time when settled communities were already flourishing in the Middle East. It is this period that is critical to our understanding of how the Parallel Roads were formed.