The Parallel Roads of Glen Roy, Scottish Highlands, represent a series of ice-dammed proglacial lake shorelines produced during the cold climate of the Younger Dryas. It has been demonstrated by Dawson, Hampton, Harrison, Greengrass and Fretwell that each lake shoreline exhibits evidence of glacio-isostatic tilting associated with the decay of the last ice sheet. The directions of tilting of the three shorelines, are at variance with published glacio-isostatic uplift isobases based on marine shoreline data that suggest a pattern of decreased uplift towards the northwest. The gradient of shoreline tilting is similar to measured regional tilts of a well-developed marine shoreline considered to have been produced in Scotland during the same period of extreme cold climate. Consideration of the ice-dammed lake shoreline data also points to the former occurrence of two separate episodes of tectonic activity during the Younger Dryas.
Historic geological investigations
In the 19th century, the Parallel Roads attracted the attention of many early geologists, including the Reverend William Buckland, Charles Lyell, James Nicol, Charles Babbage, James Geikie, and Joseph Prestwich. This interest ensured that the Parallel Roads featured prominently in the development of geological science. Charles Darwin visited the glen in June 1838 and, drawing on his recent findings in South America during the Beagle expedition concluded that the shorelines were raised beaches of marine origin. His paper on the subject was published in 1839 in the Philosophical Transactions of the Royal Society. This was contradicted in 1840 by Louis Agassiz's Glacial theory which postulated that the Glen Roy shorelines had been cut by freeze-thaw processes of loch ice during the maximum extent of glacial ice in the climatic reversal known as the Younger Dryas / Greenland Stadial, or locally the Loch Lomond Readvance. Darwin defended his paper over many years. In 1861 Thomas Jamieson was advised by Lyell and Darwin about visiting the Glen, and given memoranda and maps by Darwin. After visiting the glen a second time in the summer of 1862, Jamieson wrote to Lyell on 15 August reporting his findings. On 6 September Darwin told Lyell and Jamieson that he was convinced: "My paper is one long gigantic blunder". Jamieson's paper on his investigations was published in 1863. Four decades after Darwin's 1839 paper and shortly before his death, in a letter to Joseph Prestwich, he said that he gave up his Glen Roy theory when he read T. F. Jamieson's paper but he agreed with Prestwich's criticism of Jamieson's interpretation of the Glen Turret delta. Interest in the Parallel Roads continues to this day, both among earth scientists intrigued by the dramatic processes that shaped that landscape, and among tourists attracted by the natural wonder of the landforms.
