What is the Loch Lomond Stadial?
The final stage of the British-Irish Ice Sheet is known as the Loch Lomond Stadial, the Loch Lomond Readvance or Younger Dryas Stadial1. In Ireland, it is known as the Nahanagan Stadial. The Loch Lomond Stadial was a period of glacial readvance during the overall shrinking of the British-Irish Ice Sheet. By the end of the Loch Lomond Stadial, all the ice fields and glaciers in Britain had entirely receded, and all the ice had gone.
When was the Loch Lomond Stadial?
The Loch Lomond Stadial came right at the end of the British-Irish Ice Sheet, during the Late Pleistocene. After a period known as the Windermere Interstadial, around 14,000 years ago, where the temperature was warmer, ice had retreated to ice caps mountainous regions. It is likely that some small ice caps survived the Windermere Interstadial2. A sudden cooling of the atmosphere in the Northern Hemisphere happened around 12,900 years ago, called the Younger Dryas, which caused the ice caps to expand and form the Loch Lomond Stadial ice masses shown in the map below3. A sudden warming at 11,700 years ago caused these ice masses to recede rapidly and melt entirely1,3,4.
There has been some recent controversy about the timing of deglaciation at the end of the Loch Lomond Stadial. Some researchers favour an early deglaciation around 12,500 years ago5. But more recent work to date the timing of retreat has upheld the traditional view that the Loch Lomond Stadial ice masses lasted until the at least the end of the Younger Dryas, with some ice lasting into the beginning of the Holocene1,6.
Where was the ice during the Loch Lomond Stadial?
The map above shows the maximum extent of the readvance of ice during the Loch Lomond Stadial. We call this stage the Loch Lomond Stadial because of the large moraines that formed close to the shores of today’s Loch Lomond7. The main mass of ice, the West Highland Glacier Complex, was centred around Rannoch Moor and Fort William8. But there were smaller ice fields located in mountainous regions of Scotland, England and Wales.
The Loch Lomond Stadial in Scotland
The West Highland Glacier Complex was the largest ice mass in the UK during the Loch Lomond Stadial. It grew on the west side of the country because of the higher rainfall brought in from the Atlantic by the prevailing winds. It extended from Ullapool in the north to Loch Lomond in the south8. Plateau ice fields also existed in the Monadhliath, the Cairngorms, and the Northwest Highlands at Assynt, as well as on Skye and Mull, with smaller cirque and valley glaciers on Harris and Arran. There was also a large plateau ice field in the Tweedsmuir Hills9.
The Loch Lomond Stadial in Wales
In Wales, the Loch Lomond Stadial was mostly restricted to cirque glaciation in the mountains of Snowdonia. This area is covered in great detail in this article that explains the landforms associated with cirque glaciers. There were also some small cirque glaciers in the Brecon Beacons8. These glaciers were the farthest-south glaciers to exist in Britain during the Loch Lomond Stadial.
The Loch Lomond Stadial in England
In England, there were Loch Lomond Stadial ice fields, cirque and valley glaciers in the Lake District10, and smaller cirque glaciers in the north Pennines and the Cheviots8. In the Lake District, a plateau ice field has been suggested, which covered High Street and the surrounding area, but the evidence from geomorphology and dating is weak8. Six cirque glaciers in the north Pennines and one cirque glacier in the Cheviots have been identified, and would have formed by accumulation of snow blown from large plateau areas8.
Ice-dammed lakes during the Loch Lomond Stadial
In 1805, English geologist George Bellas Greenough visited the already-famous Parallel Roads of Glen Roy, and noted them to be similar to shorelines of a lake. He then interpreted the parallel roads to be the shorelines of a former lake, although he and later geologists could not agree on the dam that would allow the lakes to form11. In 1840, comparing Scotland to the present-day Alps, Louis Agassiz interpreted the parallel roads to be shorelines of ice-dammed lakes, using this evidence to announce there had once been glaciers in Scotland11.
Three parallel roads, one at 350 m, one at 325 m, and one at 260 m, mark different elevations of the lake surface, and three different stages of ice-dammed lake12. The first lake shoreline to form was when the ice was at its greatest extent, forming the 350 m shoreline. Then, as ice began to retreat, the 325 m shoreline formed, followed by the 260 m lake12. The lake finally drained at the end of the Loch Lomond Stadial, in a catastrophic event known as a glacial lake outburst flood, or jökulhlaup, which drained the water away into the Great Glen13.
Other ice-dammed lakes existed during the Loch Lomond Stadial too8. A large proglacial lake fomed at the terminus of the Loch Lomond glacier, called Glacial Lake Blane4. Smaller lakes existed in Glen Doe, Glen Garry, and at Achnasheen8.
Why is the Loch Lomond Stadial so important?
The Younger Dryas cold period was a time when the climate changed rapidly, first a rapid cooling at the start of the Younger Dryas, then a rapid warming at the end3. Because of this rapid change over tens to hundreds of years, we study the Younger Dryas as it may hold clues to future rapid change. We need to understand what drives ice-sheet retreat to forecast what may happen to future glaciers and ice sheets, and the Younger Dryas glaciation is perfect for this. Because it is the most recent phase of glaciation, evidence for the Loch Lomond Stadial is well-preserved, and can tell us about how glaciers and ice fields reacted to sudden warming14.
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