As old as the hills
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When ice on a slope gets thick enough it begins to deform and spread outwards under its own weight. Gravity takes over and the glacier moves downslope. The ice crystals in the glacier start to slide along internal planes, like playing cards in a deck slide over each other. At the same time the ice crystals recrystallize into new shapes, moving downslope all the time.
The size (or mass) of a glacier changes as the environmental conditions change - if there is more snow to be turned into ice, the glacier will get bigger. On the other hand, in warmer periods there will be less snow, so the glacier will melt and its front end will retreat backwards. Glaciers and ice sheets stop spreading out when they meet the ocean. This is because they start ‘calving’ or breaking off into the sea forming icebergs and ice floes.
As glaciers move they carve out the landscape. They scoop up and remove soil and weathered rock fragments as they travel across the surface. They flow because the weight of the overlying ice causes the bottom layer of ice to melt, this liquid layer then freezes onto bedrock and plucks out bits of rock as the glacier moves forward, all the time melting and re-freezing at its base. The rock fragments are trapped in the ice as it moves, acting like coarse sandpaper, grinding out even deeper crevices and valleys. This is called ‘scouring’ – it’s a process that can form all kinds of features, like glacial ‘striations’ or scratches on the rocks, which show where the ice sheet passed over them.
Glaciers especially follow pre-existing river valleys, but they make them their own by turning them into wide ‘U’ shaped valleys:
The rocks and fragments that are carried along in the base of the ice are called the ‘load’ of the glacier. This consists of boulders, pebbles, gravel and very finely ground up rock called ‘rock flour’. The load cannot be sorted out according to size like a river would do, so it is deposited all mixed up together when the glacier melts. It is either plastered on the ground or released at the glacier margins in humps and lumps which we call ‘moraine’ – you may have heard it described as ‘boulder clay’.
Ice sheets are also capable of transporting huge boulders over great distances. When the ice melts the boulders are left behind, often in regions where the local rocks are quite different. They are called ‘erratics’ – look out for them in your local area.
What happens next?
Scientists have speculated on whether we are really out of the ice age, or if we are simply in an interglacial warm period. If this is the case, when will our present interglacial end? Most interglacials last for about 11,000 years. Our current interglacial has been going on for about 10,000 years, so perhaps the end is not far away. Using evidence from the past, it could end abruptly with rapid fluctuations between warm and cold conditions, and then prolonged cold. But what about ‘global warming’, won’t that help to keep us warm? No! In fact it might have the reverse effect. Global warming could cause more rain in the northern latitudes, which could lower the salinity of the surface sea water in the northern Atlantic. This would shut down the oceanic circulation which brings warm water northwards from the equator ... the result - rapid cooling of our climate and another big freeze!
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