See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/240676796 Distribution and development of the Main Rock Platform, western Scotland: comment Article in Scottish Journal of Geology · October 1989 DOI: 10.1144/sjg25020227 CITATIONS READS 9 17 2 authors, including: Murray Gray Queen Mary, University of London 78 PUBLICATIONS 3,471 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: The Geotourism Industry in the 21st Century The Origin, Principles, and Futuristic Approach View project Geoheritage View project All content following this page was uploaded by Murray Gray on 18 January 2016. The user has requested enhancement of the downloaded file. Letter to the Editors DISTRIBUTION AND DEVELOPMENT OF THE MAIN ROCK PLATFORM, WESTERN SCOTLAND! COMMENT SIRS—Dawson (1988), in his study of the Main Rock Platform in Ardnamurchan and Moidart has discussed more general aspects of the distribution and develop­ ment of the shoreline throughout western Scotland. I wish to add to this discussion since it seems to me that the distribution and development of the Main Rock Platform are rather more complex than Dawson suggests. 1. Dawson (p. 167) suggests that the excellent development of the platform fragments in areas of limited fetch in Loch Ailort (Fig. 1), in areas where glacial erosion was formerly intense, indicates that the fragments are unlikely to have been overridden by ice. Later (p. 170), he uses the presence of these fragments and others in Loch Moidart to suggest that neither loch was occupied by glacier ice during the Loch Lomond Stadial. However, elsewhere in western Scotland well developed fragments of the Main Rock Platform occur in similar locations inside the generally agreed limits of the Loch Lomond Stadial. The locations include Corran, in Loch Linnhe (Peacock 1975) and Loch Spelve on Mull (Gray 1978). Thus care must be taken in implying the absence of Loch Lomond Stadial glaciers from the presence of well-developed Main Rock Platform fragments. 2. Dawson (pp. 161-63) also discusses the problem of the pronounced contrast in the clarity of the shoreline between the SW and NW Highlands, first pointed out by Bailey et al. (1924), who drew a line to mark the boundary between the two areas (Fig. 1). Dawson states (p. 171) that "The observed distribution of stretches of the Main Rock Platform bears no apparent relationship to rock lithology. there are relativity few platforms in the schist bedrock areas of Moidart yet where platforms do occur. they are often spectacular features. By contrast, the schists of the SW Highlands (e.g. Knapdale, Kintyre and northern Arran) are often associated with exceptionally well-developed areas of Main Rock Platform". Two points can be made about these statements. First, in the western Highlands, the Dalradian and Moinian metamorphic successions are more complex than "schist" and schist itself is not a uniform rock type in mineralogy, texture or metamorphic structure. Thus any differences in development of the shoreline that do occur in two areas of Highland metamorphic rocks could be the result of differences in the resistances of the rocks. Secondly, however, the contrast in development of the shoreline in these two metamorphic areas is not as marked as Dawson suggests. There are long stretches of the coastlines of mid-Argyll, Knapdale and Kintyre where the Main Rock Platform is absent or poorly developed. For example, on the mainland coast Scott. J. Geol. 25, (2), 227-231, 1989 228 LETTERS TO THE EDITORS between Loch Melfort and West Loch Tarbert (Fig. 1) the Main Rock Platform is only patchily developed. In a survey of this coastline (Gray 1978), the only fragments considered sufficiently well-developed to level occurred at Croaibh Haven, Craignish Castle, Keillmore and around Kilberry. Only along the eastern coast of Kintyre south of Skipness does the shoreline begin to approach continuity along the coast. Thus the pattern of distribution and development of the Main Rock Platform in the Dalradian rocks of the SW Highlands south of Loch Melfort has much in common with that in Ardnamurchan and Moidart where "although it is often exceptionally well-developed in the areas in which it occurs, it is absent from extensive stretches of coastline" (Dawson 1988, p. 171). Although complicated by several factors, the real contrast in the development and distribution of the Main Rock Platform is between the Oban/Firth of Lorn area (Gray 1974) and more peripheral locations. As" Bailey et al. (1924, p. 409) pointed out "It is only after experience has shown that the deterioration is definitely regional that one is driven to propound a regional explanation". 3. Dawson (p. 172) also discussed Peacock et al. 's (1978) suggestion that the Main Rock Platform may have resulted from the retrimming of an earlier platform. (Note that the wrong paper was referenced as Peacock et al., 1978, in Dawson's paper). He speculated that the contrast described above results from the absence of this earlier formed platform to the north of Bailey et al.'s line due the presence of an ice mass "whose southern margin was located close to the line. ". Such a glacial limit, if real, would have had a rather tortuous southern ice margin (Fig. 1). In fact the explanation for the shape of Bailey et al. 's line is much simpler. First, the contortions of the line over Morvern are readily explained by contrasts in rock resistance. The shoreline was not eroded in the massively jointed Strontian microgranite that outcrops to the south-east of Loch AUne on the southern tip of Morvern, but does occur in the Tertiary basalt of the Sound of Mull to the north-west (Wain-Hobson 1981). Secondly, the disappearance westwards across Mull is the combined result of a gradual deterioration in development (as described above) together with the glacio-isostatic tilt of the shoreline carrying it out of sight below sea-level in west Mull. In this area it is probably represented by the intertidal platforms around Gribun and Inch Kenneth, by McKinnon's Cave at Gribun and by Fingal's and other caves on Staffa (Walker et al. 1985). Thus Bailey et al. 's) line is very questionable as a glacial limit. 4. As well as the Morvern example quoted above, rock lithology is a significant factor in explaining many other contrasts in the distribution and development of the Main Rock Platform in western Scotland. A good example occurs on the Isle of Seil, south of Oban. In the south-west of the island black slate outcrops and the platform is in excess of 100 m wide with cliffs up to 25 m high over a continuous distance of 2-5 km. This contrasts sharply with the very poor development of the shoreline both to the north in the andesite around Easdale and to the south in epidiorite on the southern tip of the island. Thus Dawson's statement that the LETTERS TO THE EDITORS 229 FIG. 1. Locations mentioned in the text. Isobases (in m O.D.) for the Main Rock Platform (after Gray, 1978; Dawson, 1988) are shown by dashed lines. The solid line depicts the "N.W. limit of pronounced marine erosion" identified by Bailey et al. (1924). 230 LETTERS TO THE EDITORS distribution of the shoreline "bears no relationship to rock lithology" could be misleading. 5. Dawson (p. 172) also discusses the possibility that the regional contrast in platform development "may be attributable to Lateglacial neotectonic activity" related to "renewed downwarping" or "dislocation" associated with the Loch Lomond Stadial limits. It is difficult to discuss this point since Dawson does not make it clear why this factor should have resulted in the contrasts observed in the field. 6. Another important factor in understanding the variations in development of the shoreline is degree of exposure. The shoreline is remarkably well developed in some very sheltered locations, yet on almost every island in the Firth of Lorn area the shoreline is significantly better developed on west facing (exposed) coasts compared with the (sheltered) east coasts. This is true, for example of Kerrera, Seil, Luing, Scarba, Jura and numerous smaller isles. 7. Finally, there are some aspects of the distribution of the platform that are very difficult to explain. One of the most problematic occurs at Scarisdale on the southern shore of Loch na Keal on the Isle of Mull. Here the isobases for the Main Rock Platform (Fig. 1) show that the shoreline should occur at c. 3 m O.D. Indeed there is a platform fragment at about this altitude between Dhiseig and Rubha na Moine (NM 507 367). However, immediately north-east of this site all the coastal rocks are heavily abraded and covered by p-forms (Gray 1981). If the Main Rock Platform were formed during the Loch Lomond Stadial why was coastal erosion so spectacularly differential? A possible solution lies in the word "if above. There is some evidence that a significant proportion of the erosion of the Main Rock Platform may have occurred prior to the Loch Lomond Stadial (Gray and Ivanovich, 1988). It is suggested that a complex history would explain many of the complexities in the distribution and development of the Main Rock Platform. In these circumstances perhaps the chronological term "Main Lateglacial Shoreline" used in the title of Dawson's paper is best avoided for the present. Thus, it is clear that there are several interacting factors involved in explaining the complex distribution and development of the Main Rock Platform. Most of the above discussion, however, involves qualitative assessments. A useful next step would be a quantitative multivariate analysis of the morphometry of the shoreline fragments, and the lithological and marine characteristics likely to have been involved in its formation.