Beyond the Ice
With the fluctuations in temperature during the Ice Age came fluctuations in the extent of the ice cover. The landforms created beyond the ice are a legacy of the cold climates, as are changes in the position of the coast.
With the accumulation of so much debris on the plateaux and hill slopes, and the constant freezing and thawing of the water within it, many fascinating landforms were created and can still be seen. These forms derive from what is technically termed 'periglacial' activity beyond the ice margins and can be seen being formed in arctic and alpine areas today. The freezing and thawing of the upper levels of the ground, above permanently frozen ground below or the thawing out of an active surface layer were the formative influences.
'Blockfields' comprising large blocks of stone characterise many mountain tops and plateaux in Scotland. The finest material has been washed below the surface or has been washed or blown away altogether. The individual boulders are often rounded and associated outcrops are very regular with both horizontal and vertical joints.
Many features show how the material has moved downslope, either in an organised and well-sorted manner, or in more chaotic form, according to a combination of the speed of movement, the gradient and the type of material. Individual boulders or groups of them roll down the slope and come to rest, often ploughing into the soil and creating a wrinkle in the surface. Some are arranged into lobes with distinct step and riser forms. Soil and other coarser debris move downslope, often in the form of lobes where the material has moved to the point of some resistance and a riser has formed as a bank of turf or boulders: looking like wrinkles on jam which is about to set. More spectacular, but less frequent, is the sorting of stones into distinctive stripes downslope, with finer material in between. More widespread are debris or talus slopes, either in a uniform sheet or in cones or tongues originating from gullies and chutes. There are also areas where the entire slopes have collapsed suddenly, best seen in the Quirang of Skye and at Gribun on Mull.
Characteristic of many mountain plateaux is hummocky ground, where seasonal freezing and thawing has caused the soil surface to bulge in a regular formation. Also, there is evidence on these plateaux of the wind having blown sand and gravel, just as it does in coastal sand dunes.
Often on mountain sides there is a distinct lower limit to the forms just described; this 'trimline' marks the upper level of erosion associated with the advance of ice down a valley, trimming off the periglacial forms which were still active above the level of the ice.
On lower ground, examples of periglacial features are more difficult to see. In the gravel terraces of the river valleys, ice-wedge casts can be found where the permanently frozen ground shrank and cracked. The cracks were filled with material from the surrounding area. At a few sites, 'involutions' represent the rumpling up into folds of the soil layers through repeated freezing and thawing.