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The Tracks of a Glacier
Planet earth, in human terms, is a place of extremes of climate with a time base measured in millennia between the warmest and coldest periods. The world oscillates between ice-ages and warm periods with the later being referred to as an inter-glacial period. The last ice-age saw maximum ice cover approximately 20,000 years ago. At that time, ice-sheets covered much of the globe and glaciers descended from the mountains. Glaciers are, quite literally, rivers of ice that flow down from mountains, melting only where the ambient temperature is high enough.
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The last ice-age ended about 10000 years ago. Global temperatures increased (without any significant human “global warming" component) and the ice sheets retreated towards Polar Regions. Continental ice (except for Antarctica) only survived at more extreme latitudes and in mountainous regions where lower temperatures are encountered.
In Europe, the permanent ice level is above 2500m at medium latitudes, descending to 700m in Iceland. Above these levels, the snow that falls during the year will remain on the ground. As more snow accumulates, the weight of snow above the ground surface increases, compacting older snow into ice. Where the gradient of the slope is high enough, the ice will flow down the mountain, towards the sea, forming a glacier. Unsurprisingly, the majority of glaciers are to be found in Polar Regions, but glaciers survive all over the world, in cold, high areas.
A glacier is a slow moving transport system, conveying material from the top of the mountain down to where the snout of the glacier meets its melt water stream. An unusual demonstration of this was provided by a gentleman who became known as Ötzi, since his body was found in a glacier in the Ötztal valley on the Austrian/Italian border. When he was found in 1991, it was initially believed that the corpse was of a modern mountaineer, but it was quickly discovered that he was considerably older. His age was dated to about 5300 years ago. He died on the mountain and his body was transported to a lower altitude by the river of ice.
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Nature's Earth Movers
However, glaciers are capable of moving objects, which are considerably heavier than a human being. Rocks that fall from the mountainside because of seismic activity are plucked loose from the ground by the erosive power of the glacier or break loose because of freeze-thaw cycles can be transported great distances and turn up above strata that may be of a completely different type. Such very large boulders are called erratics and provided the earliest clues to the existence of the ice-ages mentioned above.
The observation, within the literature, that erratics were due to transportation by glaciers was credited to Pierre Martel in 1742. Martel visited the Chamonix region of France and learned from locals that the glaciers had retreated up the mountain over many years, depositing these large boulders as the ice beneath them retreated. Over time, it was theorized that the extent of glaciation in Europe (and indeed elsewhere around the world) had been much greater in the past and ice ages were first postulated.
At the height of the ice age, it has been suggested that the ice sheets, which covered much of the globe, were up to several miles thick in places. The tremendous depth of ice equates to an enormous pressure against the ground at the ice/earth interface. Consequently, glaciers have been responsible for shaping the terrain that they are found in by processes of erosion and deposition. The abrasive properties of glacial ice are enhanced by the rock debris that the glacier picks up at this interface as the glacier carves its way down the mountainside (adopting the path of least resistance, just as a river does) and on down into the valley.
Having described what a glacier is and how they transport material great distances from their apparent (current) positions, it is now time to address the question of just what is a moraine. The answer is very straightforward. Moraine is the name given to the material, which is excavated by the glacier and deposited at its margins.
There are three types of moraine, which are defined by their position: medial moraine, lateral moraine, and terminal moraine. Terminal moraine is material deposited at the snout of the glacier (its lowest altitude point) where the glacier meets its melt-water stream. Lateral moraine is to be found at the left and right hand margins of the glacier. Medial moraine is a special case of lateral moraine and is to be found at the confluence of two glaciers.
Since the glaciers have grown and retreated many times, with successive ice ages and temperate periods, deposits of moraine can be found well away from the current position of a glacier. They mark the boundaries of old glaciers, but their traces may have become obscured by the passage of time, geological or human factors.
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- Otzi, the Iceman: http://www.iceman.it/en/oetzi-the-iceman
- Glaciers: http://www.swisseduc.ch/glaciers/
- National Snow and Ice Data Center: http://nsidc.org/glaciers/