For Lyell’s letter of 22 October 1861, see Correspondence vol. 13, Supplement.

One set of phenomena critical for those who speculated about the formation of the so-called parallel roads of Glen Roy, in Lochaber, Scotland, was the existence of ‘cols’ at the head of glens, coincident in level with the various shelves. For those who viewed the shelves as the shores of former glacial lakes, these cols were watersheds or over-flow points, where the rain-water drained from the lakes. For CD, however, who held that the shelves were the remnants of marine beaches formed when an arm of the sea was present in the Lochaber glens, these areas had been ‘land-straits’ between former islands (see ‘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 of London (1839): pt 1, 39–81; Collected papers 1: 89–137). When Lyell came to write up his account of the parallel roads of Glen Roy, he retracted his previous support for CD’s theory, given in C. Lyell 1841, and adopted the glacier-lake explanation offered by Thomas Francis Jamieson. According to Lyell, these cols did indeed appear to be watersheds ‘such as would have been formed if the waters of the rivers had been made to flow east, or out of the upper ends of the supposed glacier-lakes, instead of escaping at the lower ends, in a westerly direction, where the great blockages of ice are assumed to have occurred.’ (C. Lyell 1863, pp. 259–60).

The existence of a shelf immediately between the two upper shelves in Glen Roy was an observation that favoured CD’s theory, for, as he states in the letter, no ‘col’ had ever been found coincident in level with this intermediate shelf. What CD says is not, in fact, accurate. As he himself pointed out in his 1839 paper, Thomas Dick Lauder did not note the existence of this intermediate shelf (see Lauder 1823 and Collected papers 1: 112).

David Milne Home. Milne 1847a, 1847b, and 1849.

Jamieson made two geological excursions to Lochaber, first in August 1861 and again in July 1862 (see letter from T. F. Jamieson, 3 September 1861, and Jamieson 1863, p. 240).