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The last ice sheet to completely cover Scotland (the Devensian ice sheet) began to melt and its margins retreat about 20,000 years ago as summer sunlight in the northern hemisphere increased. Over the following 10,000 years, the average global temperature rose by 3.50C and most of the Devensian ice sheet melted in the “Great Thaw”. In Scotland, however, a seemingly contradictory readvance of the ice took place. Around 12,900 years ago, the temperatures in Scotland plummeted and remained low for about 1300 years. This glacial period, known as the Younger Dryas, is thought to have been caused by huge quantities of meltwater escaping into the North Atlantic and closing down the North Atlantic Ocean Conveyor.
The evidence for this readvance was first analysed and appreciated in the broad hummocky lowlands to the south of Loch Lomond and this became the type area for the readvance .