The Ice Age
Around 2.5 million years ago the Scottish climate descended into an Ice age. The relatively stable subtropical climate of Neogene times was replaced by an unstable cold climate punctuated by short warmer episodes. About 750,000 years ago, the climate became even more severe. The extremely cold glacial periods lasted 100,000 years on average, whilst the warm interglacials were scarce and brief. Ice caps had plenty of time to accumulate and grow during these cold periods. During the most recent glacial period (the late Devensian), only 20,000 years ago, a great ice sheet covered almost the whole of northern Britain. The western Scottish Highlands were the major source of this ice.
At its maximum extent the last Scottish ice sheet overtopped all but the highest mountains in northwest Scotland. The highest summits on Rum would also have been ice-free, forming nunataks like those seen today in coastal parts of Greenland. At this time the Scottish ice sheet would have been up to 1000 metres thick on the mainland to the east of Rum.
The ice was channelled westwards through deep fjords such as Loch Broom, Loch Hourn and Loch Nevis, and streamed across the Small Isles, being deflected slightly around the mountains on Rum.
Following a sudden climatic warming around 14,700 years ago the ice sheet in northwest Scotland started to melt rapidly. The glaciers retreated back into the mountains and plants colonised the low ground. This brief warm spell lasted almost 1500 years and was comparable to the climate we know today. However, the respite was short lived. Around 13,000 years ago temperatures fell markedly and arctic conditions returned once more. This brief glacial spell (the Loch Lomond Stadial) brought small ice caps back to the high ground in northwest Scotland.
Rum nourished its own glaciers during this time, in the corries of the Cuillin and the western hills.
Only in the last 11,000 years has northwest Scotland been completely ice free. Following the last glaciation, the climate of Rum warmed extremely rapidly - summer temperatures possibly rising by 5 to 10ºC in less than 100 years! Warm, moist, interglacial conditions have prevailed on the island ever since. After the retreat of the ice, Rum was rapidly colonised by vegetation, with the development of juniper scrub followed by birch and hazel woodland.