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4. How does the ice collect and flow?
An erratic on the Aran Islands
We all know that when snow falls it collects on the landscape but in today's climate usually melts in a matter of hours. Not so during glacial conditions in Ireland. If snow begins to accumulate it forms a compact, icy substance called firn or neve. This can further compact as accumulation increases and when this material attains a depth of 50 m or so it begins to flow as blue glacier ice (similar to the manner that wet concrete flows).
As the ice moves over its substrate, small pieces of rock and soil freeze to the base of the glacier and are 'plucked' from their resting place and incorporated into the base of the glacier. This makes the base of the glacier more grating and it can further erode the underlying material, just as coarse sandpaper will on wood. Through these processes of 'plucking' and 'abrasion' the ice erodes everything it passes over, smoothing and polishing underlying rock and picking up material in its basal layers.
Rock and soil debris that is incorporated into the glacier will be carried far from their source. Rock material carried in this way is known as erratic material and the individual rocks are known as erratics. The erratics may be small pebbles or sand grains forming soil or subsoil or may be large cobbles or boulders strewn across the landscape (see photo above). The distinction of 'erratic' is the fact that they are resting on a bedrock dissimilar to their own rock type e.g. sandstone blocks resting on an area of limestone bedrock.
