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Friday, December 31, 2010
The Last Ice Age
In four main periods during the last ice age, a vast sheet of ice advanced south from the North Pole, covering Canada, Greenland, Siberia, Scandinavia and most of Britain including the North Sea. Before its course was blocked by a moraine, the River Thames used to flow far to the north of London, past St Albans. The Great Lakes between Canada and the USA are the remains of great melt water lakes. As the ice retreated the lakes filled until they drained over higher ground to the south.
When the ice retreated from the St. Lawrence, vast quantities of cold, fresh water flooded into the Atlantic, disrupting ocean currents and causing a brief refreeze since fresh water freezes at higher temperatures than salt water. The first ice ages that left their mark in the rock record occurred during the Pre Cambrian period. An ice age during the Ordovician affected what is now the Sahara desert. One in the Carboniferous period caught much of the Southern Hemisphere.
The most recent ice age began about 3.5 million years ago and is probably still in progress, though we are at present in a relatively mild spell. The temperature appears to have fluctuated between two relatively stable states about 35 times during the Earth’s history, triggered perhaps by the wobble of the Earth on its axis and variations in the Sun’s activity.