This month saw the full force of winter arrive in Edinburgh. Dubbed "The Beast from East" the UK was battered by freezing winds and saw up to 40 cm of accumulated snow. Temperatures dipped to their lowest in 30 years and transport and industry ground to a halt. However, this cold snap was nothing compared to that Edinburgh and the UK experienced during the last ice age. This month we investigate how the landscape of Edinburgh was sculpted by ice 18,000 years ago.
The Last Ice Age
Subtle cyclic changes in Earth's orbit around the sun occur naturally over tens of thousands of years, and it is these changes that have plunged the Earth into Ice Ages in the past. The last significant such event was over 15,000 years ago. At this time, the ice sheets of the Arctic expanded to many times greater than their size today, and buried much of Northern Europe. At its maximum thickness, Edinburgh was buried beneath over 1km of ice. The ice sheet extended south to blanket Northern England, Wales and Ireland.
Northern Europe looked very different to how it looks today, and Britain was joined to Europe by land. Woolly mammoths, sabre toothed tigers and even early humans lived in this tundra as the ice sheet periodically advanced and retreated. The ice sheet was very mobile, constantly 'flowing' over the landscape and grinding away the rocks it encountered.
Edinburgh: Sculpted by Ice
The map shows that Edinburgh was completely covered during the last Ice Age, and that the ice was flowing in an Eastwardly direction. This ice eroded and removed a great deal of the soft Carboniferous sediment that originally would have covered much of Edinburgh, and today only small pockets of these sediments remain (see blog post from June 2017: Carboniferous Coals and Centipedes).
The volcanic rocks of Edinburgh were, however, more resistant to erosion. The Volcanic Plugs of Arthur's Seat and Castle Rock were exposed as the soft sediment around them was scraped away. The hard rock acted as a barrier of the ice. Behind these barriers, the land was sheltered form the most erosive power of the ice, and debris (or 'glacial moraine') was deposited. The Royal Mile is one such debris tail. The resulting profile of the landscape shows a steep slope to the west, and a gentle inclining slope to the west (down 'flow' of the ice sheet). This is known as a 'crag and tail' landscape.
Other hard, more resistant rocks are found across Edinburgh. Sailsbury Crag is an intrusive sill made from hard teschenite igneous rock. Additional extrusive lavas make up the Whinny Hill in Holyrood Park, Calton Hill and nearby Craiglockhart Hill. All these hills show the same characteristics of a steep eastern slope and a more gentle western slope.
The ice sheets finally melted and associated increased sea levels marooned the United Kingdom as an island separate from the European continent. The Ice Age formed the landscape we seen in Great Britain today, and clues of its icy past can be seen in the rocks throughout the country. Thankfully, the climate has warmed to allow for habitation of Northern Europe, largely due to favourable ocean currents. Perhaps next time a big winter storm arrives, we should be thankful that we are not experiencing an Ice Age!
'Apres Geology'
It is well worth a trip up to the top of Calton Hill for the views of the Ice-formed landscape of Edinburgh. After the hike, there are some nearby attractions and bars to warm up in!
Dynamic Earth: Sitting directly below Arthur's Seat, Dynamic Earth gives more details on the geological history of Edinburgh. You can 'experience' the ice age for real!
The Sailsbury Arms: Enjoy views of the steep Sailsbury Crags from the beer garden of the Sailsbury Arms. (Maybe wait until summer though!)
Juniper Bar: For those colder days, the 'crag and tail' landscape of Edinburgh can be viewed from the Juniper Bar on Princes Street. A gin and tonic always helps the geological discussion!
-- Happy Exploring --
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