The So-Called High 'Interglacial' Rock Shoreline of Western Scotland

Abstract

It has been widely accepted for seventy years that a high (18-51 m) raised marine shoreline cut in rock in the Inner Hebrides is of 'preglacial' or interglacial age. Critical analysis of the literature and recent work on raised shorelines in Wester Ross shows that this interpretation is untenable. It is argued that the so-called shoreline is a series of isostatically tilted shorelines produced during the last and previous glacials by frost action and wave action. The new interpretation contradicts the long-held view that coastal erosion of bedrock was insignificant during high glacial sea levels. It is shown that this view is based on circular argument. In 1911 Wright described a 'preglacial' rock shoreline in part of the Scottish Inner Hebrides. He used the term 'preglacial' in the sense that the shoreline pre-dated the only apparent glaciation of the area. More recently, following McCann (1964), the shoreline has often been referred to as 'interglacial', an interpretation that has been widely accepted. The paper proposes that this interpretation is incorrect. Although it has been recognized that the interglacial shoreline may be the result of more than one period of marine erosion (e.g. McCann, 1968), it is normally referred to as a single shoreline. Hence this custom is followed here initially. 'High' is placed before 'interglacial' since the feature as described in the literature is at 18-51 m O.D., and since an intertidal rock shoreline interpreted as interglacial in age has been described in western Scotland (Dawson, 1980a). DESCRIPTION OF THE SHORELINE The high interglacial shoreline has been reported as occurring on Colonsay, Oronsay, Islay, Jura, the Treshnish Islands, Mull, Ulva, Gometra, Iona, Rhum, Skye and Raasay, all in the Inner Hebrides, as well as on the mainland coast of Ardnamurchan and Applecross (Wright, 1911; Craig et al., 1911; Bailey et al., 1924; McCann, 1964, 1968; Richards, 1969; McCann and Richards, 1969; Dawson, 1979a). In these areas there occurs occasionally or extensively a raised rock platform backed by a fossil cliff. Frequently the front of the platform is terminated by a cliff that descends to a lower raised marine feature or to the present shore. In north-east Skye the platform is up to 200 m wide. In west Mull the feature extends for many kilometres, the backing cliff being nearly 90 m high at one place. The platform is especially obvious on the Bac Mor, one of the Treshnish Islands, which is commonly called the Dutchman's Cap: here the platform occupies more than two-thirds of the island, surrounding a steep-sided central hill that rises nearly 60 m above it. In Colonsay the platform attains a maximal width of some 800 m and in north Islay exceeds 1 km. The platform in Colonsay and Oronsay is said to be everywhere ice-moulded and in Trans. Inst. Br. Geogr. N.S. 7: 205-216 (1982) Printed in Great Britain This content downloaded from 157.55.39.35 on Wed, 31 Aug 2016 04:34:09 UTC All use subject to http://about.jstor.org/terms