EPISODE:

GLEN ROY, 1839

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“It is a startling assumption to close up the mouth of even one valley by an enormous imaginary barrier; to do this with all would be monstrous . . . of such barriers . . . I need not say there does not exist any trace. . . It is perhaps useless to ask, were the barriers composed of rock or alluvium?”

Charles Darwin (1808-1882) studied the “Parallel Roads” in 1838 and published is views in 1839. Darwin at this time was becoming well known as a geologist. He had returned from the Beagle voyage in 1836 and in 1835 and 1837 had published observations on recent uplift of land in South America and in the Pacific basin. His observations on the Beagle voyage supported the view of Charles Lyell (1797-1875) that mountain uplift and the interchange of land and sea were due to gradual changes in the elevation of land. Lyell argued that “causes now in operation” were sufficient to accomplish changes such as this (i.e. there was no need to resort to sudden and catastrophic events at all, beyond those observable now in the world). Lyell, who was famous for his three-volume “Principles of Geology,” which re-kindled Darwin's interest in geology as he read it on the Beagle, believed that Earth had existed for a great length of time and had done so in essentially the same overall state as it was presently to be found. William Whewell (1794-1866), the philosopher of science, termed Lyell’s views “uniformitarian.”

Darwin’s map of the Parallel Roads of the Lochaber region

In the “Principles” Lyell catalogued a great many observations of geological processes observed in the world today and their resulting changes in Earth’s surface. Darwin’s work in South America fit well with this uniformitarian philosophy, and Darwin was a convert.

Darwin interpreted the parallel roads as marine in origin. As in South America, he believed this transgression was gradual and reflected, at different times and places, fluidity in the underlying Earth. However, to accept this explanation he had to explain away the lack of marine shells in the beaches (the Parallel Roads), their lack of interconnectedness to the coast in this area, and their lack of universality in the country as a whole. Of MacCulloch’s earlier hypotheses of a lakes being dammed in the valleys by some unknown agent, he wrote (p.55): “It is a startling assumption to close up the mouth of even one valley by an enormous imaginary barrier; to do this with all would be monstrous. Of such barriers in the district we are considering I need not say there does not exist any trace.”

Darwin also noted the presence of erratics in the area. He reviewed the deluge and drift theories and favored the drift theory of Lyell because, among other things, the beaches could not have survived a deluge. He attributed the origin of the ice bergs to glaciers that occupied the mountains at this time, “when its climate was more equable (chiefly consequent on the larger area of water), which favours a low limit of the snow line, and therefore the probability of glaciers, the parents of icebergs, descending in favourable places into the sea. It is therefore to this period, if this view be correct, that we must refer the 'parallel roads of Lochaber,' and consequently the elevation of the land . . ."

Concerning striations: “This question will recall to the mind of those who have read the late papers of Messrs. Charpentier, Venetz, and Agassiz, the case of the longitudinally and obliquely scratched rocks of the Alps. In the Addenda to my Journal during the voyage of the Beagle, I have endeavoured to show that the passage of ice [bergs], with imbedded fragments of rock, acting at successive levels on the surface of shoals during the gradual rising of the land, offers the most probable explanation of the scratches and grooves, which have justly excited so much attention in Scotland and other places.”

Darwin published his work on Glen Roy’s parallel roads in 1839: it was shortly to be overtaken by a new theory of their fresh-water origin. Much later, Darwin described his work on the Parallel Roads as a “gigantic blunder.”

QUESTIONS:

1. If the beaches were of marine origin, what organic evidence might you expect to find in their deposits? Did Darwin find such evidence?

2. If you were a "uniformitarian" would you be inclined to accept ice-sheet glaciations as an explanatory theory for beaches, erratics, etc.?

3. Is a uniformitarian view derived from observations or is it an assumption and an application of a method?

REFERENCES

Darwin, C. 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 Transactions of the Royal Society of London, for the year MDCCCXXXIX, Part I, p.39-82 http://www.google.com/books?id=NfMAAAAAYAAJ&pg=PA39

Ice-drifted “drop stones” similar to this are found in laminated water-laid sediments in Glen Roy.
http://upload.wikimedia.org/wikipedia/commons/thumb/c/c9/Dropstone.JPG/800px-Dropstone.JPG

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