Although Darwin was best known for his geological work in South America and other remote Beagle destinations, he made one noteworthy attempt to explain a puzzling feature of British geology. In 1838, two years after returning from the voyage, he travelled to the Scottish Highlands to study the so-called parallel roads of Glen Roy.
These ‘roads’ were horizontal terraces on either side of a valley called Glen Roy, and though earlier visitors had supposed that they must be ancient hand-built features, geologists in the last two decades had declared them to be of natural origin. Two Scotsmen, John MacCulloch and Thomas Dick Lauder, proposed in the late 1810s that the roads had been cut into the hillsides by standing water, and were the beaches of a former highland lake that had once filled the valley. They supposed the water in the lake to have stood at several distinct levels, each corresponding to the level of one of the roads.
Darwin’s interest in the parallel roads was piqued by his previous study of a series of terraces at Coquimbo, Chile, which he believed were former marine beaches that had since been pushed above sea level by the bulging of the earth beneath South America. He went to Scotland in hopes of demonstrating that the Glen Roy roads were also former sea beaches. If this were the case, their existence would indicate that Scotland had been elevated from the sea in a manner similar to the process he believed had lifted the continent of South America. In each case, the fact that the terraces remained essentially level indicated to Darwin that tectonic movements could be gradual and equable (as the upright pillars of the temple at Serapis had famously suggested to Charles Lyell).
In 1839 Darwin read a paper on the parallel roads to the Royal Society of London. He dismissed the notion that they were former lake beaches on the grounds that there was no satisfactory explanation for the temporary damming of Glen Roy, which must have occurred for the valley to fill with water and then be emptied. Instead, he advanced his theory: ‘the whole country has been slowly elevated, the movements having been interrupted by as many periods of rest as there are shelves.’ The roads were of marine origin, and each road represented a former stage in Scotland’s emergence from the sea.
While Darwin was thus able to avoid conjecturing about an event that could have dammed Glen Roy, he instead had to explain why the sea had left no marine fossils on the sides of the glen and why it had not cut similar terraces on other hillsides across Scotland. He argued that the preservation of both fossils and old sea beaches should be considered the exception rather than the rule. For instance, Darwin pointed to a number of locations, ranging from his home county of Shropshire to the coasts of Scandinavia, where exposed deposits of undoubted marine origin had been found not to contain any marine shells, presumably because they had been dissolved by acidified rain. Likewise, he pointed out that durable terraces like the roads might have been formed only where a special combination of currents and tides were acting on a coastline of a particular geological composition.
Scarcely had Darwin’s Glen Roy paper appeared in print than the Swiss geologist Louis Agassiz proposed an explanation for the roads that had not been considered by Lauder, MacCulloch, or Darwin. Agassiz was convinced that the earth had formerly experienced an ‘epoch of great cold’, and that glaciers had once been much more widespread across Europe. In 1840 he toured locations in Britain with many leading geologists, pointing out how many familiar phenomena could be reinterpreted with reference to the former action of glaciers. In the case of Glen Roy, Agassiz provided the missing component of the lake-beach theory of the formation of the parallel roads. A wall of ice extending across the foot of the valley could have dammed Glen Roy and formed a glacial lake like those seen in the present-day Alps.
In the hands of Agassiz and others in the succeeding decades, glacial theory prompted geologists to reappraise much more than the terraces at Glen Roy. Darwin was resistant to the glacial explanation for the parallel roads, even as he admitted the action of ice sheets elsewhere. On his last ever geological field trip, a return visit to North Wales in 1842, Darwin wrote that the signs of glacial action in the valley of Cwm Idwal could not have been more obvious ‘if it had still been filled by a glacier.’ Yet in letters written as late as 1861, Darwin continued to defend, albeit halfheartedly, the marine theory of the formation of the parallel roads (see sidebar to the right). Darwin was later to write, notoriously, in his autobiographical ‘Recollections’ that his paper on Glen Roy was a great failure: ‘and I am ashamed of it.’
Although Darwin eventually abandoned his original conclusions about Glen Roy, it is well worth trying to retrace Darwin’s footsteps there. To understand what led Darwin to ‘see’ what he saw in 1838 is to take a glimpse from the perspective of the young geologist when he was giving full expression to the theory of the earth that was his proudest product of the Beagle voyage.