The Val de Bagnes runs along the river Drance to the south-east of Martigny in the canton of Valais, Switzerland. Lyell refers to the ice and snow that commonly fell from the mountainsides and accumulated, impeding the flow of the Drance near Mauvoisin. In June 1818, after an exceptionally large accumulation of ice the previous winter, the lake that was formed above Mauvoisin burst through the ice barrier, causing devastation to the entire Val de Bagnes (Baedeker 1889, p. 292).

For many years, the cause of the famous parallel terraces or ‘roads’ of Glen Roy in the district of Lochaber, southern Inverness-shire, Scotland, had been debated. For CD’s theory of the marine origin of the parallel roads, published in 1839, see ‘Parallel roads of Glen Roy’; see also Rudwick 1974. CD corresponded on the subject with Lyell and Thomas Francis Jamieson, after Jamieson’s visit to Lochaber in August 1861 (see Correspondence vol. 9, Appendix IX). Jamieson became convinced that the roads represented the shores of former glacial lakes and that the level of these shores coincided with the level of various ‘cols’ or passes at the head of valleys (glens), indicating that the cols had once been points of outflow. Crucial to his hypothesis was the proposal that glacial barriers had at one time existed at the mouths of the glens. In a letter of 14 October 1861, Jamieson suggested to Lyell that the barriers of the Glen Roy lakes were ‘mainly of ice’ (Correspondence vol. 9, Appendix IX). After reading Jamieson’s letter, CD had expressed doubts about the possibility of a ‘mere barrier of ice’ being able to account for the formation of shelves at Glen Roy (see Correspondence vol. 9, letter to Charles Lyell, 20 October [1861] and n. 1). Lyell, by analogy with the case of the Swiss ice barrier, evidently concluded that ice barriers in Lochaber would have given way under the force of the water or, if the ice were continually increasing, have resulted in a constantly changing depth of water. This argument was based on the supposition that the ice barrier would have been at a lower level than the col so that water would overflow the barrier rather than the col.

David Milne had found a col at Glen Glaster, at the same level as one of the shelves, which had been missed by previous observers. He also mentioned two intermediate shelves, faintly discernable, which he took as evidence of the gradual lowering of a barrier (Milne 1847, pp. 398, 404).

Land-strait: ‘isthmus’ (OED).

Lyell evidently sent this letter to Jamieson, who, in his reply, agreed with Lyell’s conclusion that cols had determined the location of the shelves and that water had overflowed the col rather than the barrier. He dismissed the conflicting evidence of intermediate shelves described by Milne, noting how faint these markings were (see Correspondence vol. 9, Appendix IX, letter from T. F. Jamieson to Charles Lyell, 27 October 1861).