"It is far the most remarkable area I ever examined… I can assure you Glen Roy has astonished me.”

Charles Darwin

So wrote the great Charles Darwin to his friend and fellow scientist Charles Lyell on 9th August 1838.  His reaction to the landscape of Glen Roy is all the more remarkable when you consider that Darwin had already voyaged around South America on board the Beagle, visiting Brazil, Patagonia, Tierra del Fuego, and the Galapagos Islands.  But he made it quite clear that he’d witnessed something extraordinary during his five days in the Highlands:  “Not even the first volcanic island, the first elevated beach, or the passage of the Cordillera [of the Andes], was so interesting to me, as this week.”

What had Darwin seen, to arouse such intense curiosity and delight?  The answer is the Parallel Roads of Glen Roy.  Even today, it’s easy to imagine his astonishment.  As you progress up the glen, the willows, alders and birches give way to bracken and open hillside, and the ‘roads’ begin to reveal themselves as bold lines running horizontally and in parallel along the flanks of the hills.  They are so level that they look like hand-drawn contours, but up close you can see that they are actually narrow shelves, made all the more noticeable by the differing growth of vegetation on and below them.  They occur at three distinct altitudes:  260, 325 and 350 metres.

What on Earth caused them?  In times gone by they were interpreted as ancient paths, built to aid travel around the steep hills.  Many places in Scotland are associated with the warrior-gods of Irish legend, and the Parallel Roads were said to be the tracks followed by Fionn MacCumhaill and his renowned huntsmen, the Fianna.  Later, they were ascribed to the kings of Scotland, who - it was said - would ride along these conveniently-placed tracks on their hunting forays out of nearby Inverlochy Castle. 

But when Charles Darwin looked at the Parallel Roads, he wasn’t seeing in his mind’s eye the flashing swords of Fionn and his ferocious warriors, or hearing the hoof-beats of King James’ horses.  He wasn’t even seeing the hand of a creator-god, which in itself was shocking to a large proportion of his generation:  in the early 1800s it was still widely accepted that the landscape had been shaped by a Biblical flood.   Darwin thought differently, and was nurturing his own revolutionary ideas.  In particular, as he wandered along the Parallel Roads, he was seeking evidence that would support his personal theory about their formation.  He was looking for seashells - or, more precisely, marine fossils.

Darwin failed to find any signs of sea life on the slopes of Glen Roy, but he wasn’t at all discouraged.  He assured Lyell: “I have fully convinced myself… that the shelves are sea-beaches,—although I could not find a trace of a shell, & I think I can explain away most, if not all, the difficulties.”  He had seen landforms in Chile which he believed were former marine terraces, proving that the land had been gradually pushed up above sea level.  He thought that in Glen Roy he had detected a similar process, and that each level of the Parallel Roads “represented a former stage in Scotland’s emergence from the sea.” 

Darwin was very persuasive, and of course he had a formidable reputation.  On this occasion, however, he was wrong.  In October 1840, just two years after Darwin’s visit, a Swiss scientist named Louis Agassiz inspected the Parallel Roads in the company of an English geologist called William Buckland.  Armed with his first-hand experience of Alpine glaciers, Agassiz envisioned something completely different.  To his friend Robert Jameson, Regius Professor of Natural History at Edinburgh University, he wrote:  “…at the foot of Ben Nevis, and in the principal valleys, I discovered the most distinct morains and polished rocky surfaces, just as in the valleys of the Swiss Alps, in the region of existing glaciers;  so that the existence of glaciers in Scotland at early periods can no longer be doubted.”

Glen Roy

Glaciers in Scotland?  How outrageous!  It was the first time anyone had seriously considered the idea.  This daring proposition was announced to the public on 7th October 1840, in The Scotsman newspaper.  Agassiz’ letter to Jameson had been intended for publication in The Philosophical Journal, but it was just a couple of days too late.  Jameson, recognising the “great importance” of his friend’s findings, sent it to the newspaper. 

Agassiz’ evidence for glacial activity didn’t stop at the moraines and “polished rocky surfaces.”  He believed that the Parallel Roads marked the former water levels of a large glacial lake.  He had seen examples of such lakes in the Alps, and he believed that a wall of ice had once blocked the head of Glen Roy.  It had formed a natural dam, stopping the water in the glen from draining away until it could overflow elsewhere. 

“I remain firmly convinced they are marine beaches,” declared Darwin in a letter to Charles Lyell in March 1841.  He held stubbornly to his misplaced idea for another 20 years, even as more scientists, including Thomas Jamieson and James Geikie, visited Glen Roy and refined Agassiz’ theory.  In October 1862, Darwin finally admitted defeat.  “I was never before fully convinced of the land glacialisation of Scotland,” he told Lyell, with a mixture of exasperation and regret.  He continued:  “Now & for ever more I give up & abominate Glen Roy & all its belongings.  How many have blundered over those horrid shelves!” 

Profile of Roads - Glen Roy

It wasn’t until the 1970s that more detailed research revealed the full extent of the Parallel Roads and their associated glacial features, not just in Glen Roy but in the adjacent Glen Gloy and Glen Spean.  But it was Louis Agassiz, the visionary who saw the the processes of glaciation with such clarity, who provided the initial spark.  As Sir Archibald Geikie observed in his book, ‘The Scenery of Scotland’ (1865):  “The ‘roads’, so long a subject of wonderment and legendary story among the Highlanders, and for so many years a source of sore perplexity among men of science, seem at last to be understood…  Instead of tracing back their origin to the days of Fingal, they stand before us as the memorials of an infinitely vaster antiquity - the shores, as it were, of a phantom lake, that came into being with the growth of the glaciers, and vanished as these melted away.”

We now know that, as the glacier in Glen Spean advanced and cut off more and more escape routes for the water, the level of Glen Roy’s lake rose from 260 to 325 and then 350 metres.  The intervals were long enough to allow the formation of shallow shorelines - the ‘roads’ - through weathering and wave action.  As temperatures warmed at the end of the glacial period, some 11,500 years ago, the water may have burst suddenly through the ice-dam, perhaps even floating the glacier off its bed with the force.  Glen Roy has retained other glacial features such as impressive alluvial fans and moraines. 


Reference and further reading:

Douglas Peacock, John Gordon, Frank May, Glen Roy - a Landscape Fashioned by Geology (SNH)

Alan McKirdy, John Gordon & Roger Crofts, Land of Mountain and Flood - the Geology and Landforms of Scotland

The Darwin Correspondence Project (Univ of Cambridge) https://www.darwinproject.ac.uk/commentary/geology/darwin-glen-roy

The Geological Society https://www.geolsoc.org.uk/GeositesGlenRoy

The Scotsman, 7th October 1840

James Geikie, The Great Ice Age and its Relation to the Antiquity of Man (1874)

David J A Evans and James D Hansom (1991) Scottish Landform Examples - The Parallel Roads of Glen Roy, Scottish Geographical Magazine 107:1

Archibald Geikie, The Scenery of Scotland (1865)