The 'Ice Age'
The Ice Periods
During the 2.4 million year period commonly called the 'Ice Age', Scotland experienced a succession of cooler and warmer periods (glacials/stadials and interglacials). With these came the accumulation of ice in the form of ice caps and glaciers. Their movement, and that of the meltwater rivers associated with them, has had a significant role in fashioning the detail of the landscape we see today. Overall, however, the basic elements of the highlands and lowlands have remained as in pre-glacial times, particularly in the south and east.
The climatic changes were often abrupt, measurable in decades. The weight of ice caused much of the country to be lowered; at the same time the capture of water in the ice meant that sea levels were lower. When the ice melted, vast quantities of water were released, depositing sands and gravels in river valleys and offshore. The melting also caused the sea level to rise and beaches were formed above the present sea level (now called 'raised beaches').
Three stages of glaciation in Scotland and Europe:
- 29 - 22,000 years ago with Scandanavian and Scottish ice sheets joining
- 15 - 14,000 years ago with an ice cap covering most of Scotland
- The Loch Lomond Stadial around 10,500 years ago
It is fair to assume that all of Scotland has been covered by ice at some stage during the last 2.4 million years. The ice caps and glaciers of the earlier cold periods, were undoubtedly most influential in eroding and reshaping the landscape. During the latest cold episode, from 115,000 to 10,000 years ago, there were probably several glacial stages. At the time of the last ice sheet, some hills in the West and Northwest Highlands could have stood out as 'nunataks' above the ice. The last cold period, termed the 'Loch Lomond Stadial' (the landforms were first described at the south end of Loch Lomond), occurred 11-10,000 years ago, and left moraines and outwash terraces in many valleys and small moraines in many corries.
We know a great deal about the effects of ice on the landscape but little of the timescales, except for the most recent events during the last 30,000 years or so as illustrated in the three maps. Ice has fashioned the landscape through erosion, smoothing and moulding the bedrock, and transporting and depositing rock debris.
Ice smoothed bare rock; it removed weathered debris and helped expose the resistant tors; it created hollows or corries (some 500 of them in Scotland); it overrode the land between river catchments, breaching the watersheds (aiding later changes in the courses of rivers); it deepened valleys to form rock basins now occupied by freshwater lochs or more particularly the sea lochs of the west coast (more than 100 of them); it also smoothed rocks on the upstream side and plucked rocks on the downstream side to form 'roches moutonnées' and smoothed others to form streamlined rock outcrops such as 'crag and tail' forms. By far the greatest impact was in the West and Northwest Highlands, illustrated by the fact that 90% of the corries and more than 90% of the rock basins are in westerly flowing river systems. There rain and snowfall were greater, the ice thicker and the glaciers steeper and faster flowing.
Ice is thought to have had a relatively benign effect on the landscape in some areas, especially in Northeast Scotland. Many of the features of the pre-ice period, especially gently rolling surfaces and weathered bedrock, remain little modified, tors have been uncovered but not destroyed and river systems remain largely intact.
Ice carried debris
Ice is not a uniform solid and has many cracks (which means that parts of it move at different rates). Rocks and rock debris of all shapes and sizes (including tiny pieces known as 'rock flour') are frozen onto the surface, within its layers and at its base. It can carry rocks long distances before depositing them and from these 'erratics' we can tell from which direction the ice moved and how far it travelled.
Ice deposited material
Ice deposited material on the ground beneath it, when its speed was reduced and where valleys widened. Where it reached the limits of its extension it deposited 'moraines'. In places, 'hummocky' moraine in the form of hundreds of hillocks was deposited on valley floors. On lower ground, where it had less momentum, it deposited sheets of material traditionally called 'boulder clay' as it often comprises stones and boulders in a fine-grained matrix (more technically called 'till'). These deposits are perhaps the most significant of all because they are so widespread in the lowlands and upland valleys and provide the 'parent' material for many of our modern soils.
Meltwater cut valleys
When ice melted it created very powerful meltwater rivers. These often followed the existing river systems, but high-water pressure can make rivers in and under the ice flow uphill, and across cols and spurs between river systems. Many of these meltwater valleys are now left dry or have streams much smaller than the valleys in which they flow. At times the meltwater spilled into valleys dammed by the ice and formed lakes whose shorelines in places like Glen Roy and Glen Gloy are still evident as the well-known 'Parallel Roads'. The lakes drained periodically under the ice in great floods.
Meltwater deposited debris
It is the deposits from these meltwater streams which are most apparent in the landscape. At the side of the ice, 'kame' terraces were deposited; under the ice (usually in tunnels), sinuous ridges or 'eskers' were formed (e.g. at Carstairs) and beyond the ice margin the debris was laid down in the form of 'outwash' plains or lake deltas (e.g. at Achnasheen). The vast extent of these features in the glens and fringing the mountain masses in the east and south of Scotland, emphasises the contrast between the landscape created by the glacial erosion in the west and by deposition in the east.