Issues

A.G. Dawson writes: The paper by Peacock (2008) represents a valuable contribution to our understanding of the Late Quaternary history of Islay in particular and the SW Scottish Hebrides as a whole. As part of the conclusions based on examination and dating of three cores (two from Loch Indaal and one from between SE Islay and SE Jura) Peacock argues that his interpretation is difficult to reconcile with the view presented by this author that the limit of the Late Devensian ice sheet lay SW–NE across Islay and is represented, in part, by the Central Islay moraine (Dawson 1982). The latter view is not a new one. For example, Sissons (1981) argued on several grounds (mostly on the basis of raised shoreline evidence) that the last (Late Devensian) Scottish ice sheet may have terminated amidst the Inner Hebridean islands. Of course, conventional wisdom would suggest that the limit of the last (Late Devensian) ice sheet lay far to the west of the Inner Hebrides, so why should an idea exist that this ice limit may have been located closer to the Scottish mainland? The initial discussion of this topic was presented by Dawson (1982) with new data described in several later papers (Benn & Dawson 1987; Dawson et al. 1997; Dawson & Dawson 2001). The interpretation of the regional deglacial history is based on geomorphological and stratigraphical evidence not mentioned in Peacock (2008). …

J.D. Peacock replies: I am aware of the considerable contribution that Prof. Dawson and his co-workers have made to the elucidation of the Late Quaternary history of Islay, but I doubt that the evidence for their hypothesis, that the western part of the island and the Mull of Oa were ice-free during the Dimlington Stadial (DS), is sufficiently robust to refute the ‘conventional’ view that Islay lay within the Last Glacial Maximum (LGM) (22–26 ka bp).

1. The idea that the high-level glaciomarine deposits in the Rhinns (up to 80 m above OD) antedate the LGM is based chiefly on the three luminescence dates referred to by Dawson. The muds from which the dated samples were collected were presumably deposited from turbid meltwaters in which the rapid rate of deposition in a marine environment would have been accelerated by flocculation of the particles. Thus opportunities for ‘resetting the clock’ by sunlight would have been extremely limited, and for this reason alone the dates are suspect, and likely to be much too high. Moreover, recent improvements in dating of muddy sediments have resulted in much lower ages compared to those published earlier (e.g. Duller 2006). There is thus good reason to believe that redating the Islay muds would provide much lower ages.

2. The intensity of frost shattering and weathering in Islay as a whole is low, and similar to that within the LGM elsewhere in Scotland. In the Rhinns the bare bedrock surfaces above the level of the glaciomarine …