Darwin thought the parallel “Roads” of Glen Roy represented vanished marine shorelines, one above the other as the result of vertical movement. Agassizexplained them, rather, as successive shorelines of a glacial lake, now vanished because the retaining glacier has melted away. If so, and if global warming is real, we might expect to see vanishing lakes today, as the glaciers retreat. We can, and we do, as a recent blog post by my friend Peter Hess explains.
Glen Roy is a valley in the Western Scottish Highlands, just south of the Great Glen (home to Loch Ness), and draining through Glen Spean to Loch Linnhe, an inlet of the Atlantic. It is remarkable for the presence of the Roads, a series of parallel, almost horizontal, grooves in the hills on the sides of the glen. Clearly shorelines; but of what body of water? And why are there more than one of them?
From Darwin, C. R. 1839. Observations on the parallel roads of Glen Roy, and of other parts of Lochaber in Scotland, with an attempt to prove that they are of marine origin. Philosophical Transactions of the Royal Society 129: 39-81, through Darwin Online
Charles Darwin visited the area in 1838, two years after his return from his round the world voyage on the Beagle. During that voyage, he had examined the geology as well as the plants and animals of the places he visited, and among them was the coastal area of Chile. This is marked by raised beaches inland where once had been shoreline, and Darwin correctly described these as the effects of uplift, which we now know to be driven by plate tectonics. So it was natural thatDarwin should have applied a similar explanation to the Roads, suggesting that the Cairngorms, like the Andes, were a zone of uplift, and that the Roads were ancient beaches of the Atlantic, now some ten miles away. The alternative theory, that they represented shorelines of an ancient lake, ran up against a seemingly conclusive objection; such a lake could only have formed if there had been a barrier across the valley, but there was no trace of this.Only a year later, shortly after going public with his Ice Age theory, the naturalist Louis Agassiz visited the area. In the Highlands he found plenty of evidence to support his idea; scratches on bedrock caused by the passage of glaciers, erratics (boulders far from their parent rock formations), and moraines (piles of rock rubble that had been carried by glaciers, left in place when the glacier melted). He considered the Roads further evidence of this; yes, there had been a lake, and yes, the roads did represent the shorelines at different times, carved into the sides of the valley by fierce freeze-thaw cycles. As for the barriers holding the lake in place at different levels over the course of time, they were a series of long vanished glaciers.
We now know that Agassiz was basically correct, although we now speak of a series of glaciations rather than a single Ice Age, and although Darwin was right in this; that the area has in addition experienced uplift, as the weight of ice above it has melted away.
Later Darwin was to write of this as his greatest blunder, describing in his Autobiography how in Wales he had missed the evidence of glaciation all around him, and generously acknowledging Agassiz for having come up with the correct explanation.
Agassiz rejected Darwin’s concept of evolution when it was published twenty years later because he believed in the fixity of species, but this does not seem to have diminished Darwin’s respect for him. What is now nothing but a deliberately cultivated ignorance was then, with so much less evidence available, no more than an understandable conservatism.
The overflow channel through which the vanished Loch Roy must have drained can still be detected as an abrupt narrow valley in the surrounding hillsides. The draining of the vanished lake in South America sent a surge through its own channel, down Chile’s main river, and caused giant waves as far as the Pacific Ocean, 60 miles away.
Lake to sandy valley overnight (from Peter Hess posting on NCSE blog site)
The glaciers of Switzerland are receding. Those of the southern Andes are receding even faster. Since Agassiz and Darwin examine the roads of Glen Roy, the earth’s mean surface temperature has increased by roughly 1oC, with another 0.5oC in the pipeline even if emissions were to be stabilised at the same levels as in the year 2000.
Which, of course, they won’t be.