Gordon, J.E. and Sutherland, D.G. GCR Editor: W.A. Wimbledon. 1993. Quaternary of Scotland. Geological Conservation Review Series No. 6. JNCC, Peterborough, ISBN 0 412 48840 X.
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A.G. Dawson
The coastal area of western Jura contains a remarkable assemblage of raised shoreline landforms. These include shore platforms and the best-developed spreads of Lateglacial shingle ridges in Britain which provide valuable information for understanding changes in relative sea level. The area is also noted for a medial moraine formed by the Late Devensian ice-sheet.
This site comprises a c. 37 km long stretch of the west coast of Jura, between Glengarrisdale Bay
There is only a limited amount of published information on the Quaternary features of this area. The raised shingle spreads were first described by officers of the Geological Survey (Wilkinson, 1900, 1907; Peach et al, 1911), and Ting (1936, 1937) gave further details of the ridges. The first major study of the raised beaches was by McCann (1961a, 1964), who sought to describe and explain the origin of the western Jura shingle spreads and relate them to patterns of Lateglacial relative sea-level change. In a later paper, McCann (1968) extended his discussion to include the raised coastal rock platforms. More recently, the raised shorelines of western Jura have been investigated in detail by Dawson (1979a). The results of this research are published in a number of later papers (Dawson, 1980a, 1980b, 1982, 1983a, 1984, 1988b, 1991).
A high rock platform and associated cliffline are almost continuous between Shian Bay
The western Jura platform pre-dates one period of general glaciation, since at two locations it is separated from the overlying raised beach sediments by lodgement till. First, at Bhrein Port
The rock platform described above is considered to correlate with the classic High Rock Platform of northern Islay (Dawson, 1979a). The High Rock Platform has not been found, however, along the intervening south-west Jura coast probably due to the presence in that area of great thicknesses of till.
The High Rock Platform of the southern Inner Hebrides (see also Northern Islay) was first described by Wright (1911). Although he did not describe the Jura platform, Wright considered that the platform was 'pre-glacial' in age. Later, McCann (1968) provided the first description of the western Jura platform and suggested instead that it was 'interglacial'. Dawson (1979a, p. 161) accepted an interglacial origin for the western Jura (and also the northern Islay) platform but considered that the shoreline had been warped by neotectonic activity. A contrary view was expressed by Sisson (1982b), who proposed that the platforms that comprise the High Rock Platform were produced by cold-climate shore erosional processes and that the features exhibit glacio-isostatic tilting. On that view, the various platform fragments are part of a series of tilted shorelines. However, the available altitudes from the west coast of Jura and northern Islay do not demonstrate any tilt to the platform, which is therefore regarded as a single feature. Thus there is at present no general agreement on their origin and age. Dawson (1983a) has argued on several grounds that formation of the western Jura platform by cold-climate shore erosion would have taken a minimum of 8000 years. Relative sea-level stability of such duration during a single period of cold climate is unlikely due to glacio-isostatic instability of the land surface and glacio-eustatic changes in sea level. It would therefore appear that the western Jura High Rock Platform represents the product of several periods of Pleistocene coastal erosion.
Between Shian Bay and Ruantallain the seaward edge of the High Rock Platform forms the cliffline of a lower platform 50–150 m wide
The platform constitutes part of a glacio-isostatically tilted shoreline that declines in altitude to the south-west, from 6 m OD in northern Jura to sea level in northern Islay (Dawson, 1980b). The shoreline gradient is 0.13 m km' and this, together with its rock-cut nature, general altitude and freshness of form, indicates correlation with the Main Rock Platform (Gray, 1974a, 1978a). McCann (1968) suggested that this feature exhibited evidence of glaciation, although Dawson (1980b) did not report such evidence, and considered that McCann's evidence for glaciation related to the Low Rock Platform (see below). The origin and age of the Main Rock Platform in western Scotland have been discussed at great length in numerous publications (for example, McCann, 1968; Sissons, 1974d; Gray, 1978a, 1989; Dawson, 1979a, 1980b, 1983a, 1988a, 1989; Sutherland, 1984a). However, the occurrence of the regionally tilted Main Rock Platform in western Jura is of particular significance since it is in this area (and also northern Islay) that this platform merges with and crosses the regionally horizontal intertidal Low Rock Platform (Dawson, 1979a, 1980a) (see below).
In south-west Jura, low intertidal rock platform fragments are conspicuous along long stretches of coast. These are typically 100 m wide and are best developed on the foreshore between Rubh'Aird na Sgitheich
Intertidal ice-moulded rock platforms also occur in north-west Jura, northern Islay and neighbouring Colonsay and were first described by Wright (1911) as representing a '… preglacial plain of marine denudation …'. Dawson (1980a) referred to the Jura and Islay feature as the Low Rock Platform and explained its regional horizontality as having been produced by marine processes during interglacials. The shoreline is also of considerable significance since the glaciated platform fragments of south-west Jura, northern Islay and Colonsay were considered by McCann (1968) to demonstrate glaciation of the Main Rock Platform.
The coastal zone of western Jura is dominated by conspicuous raised beach terraces and 'staircases' of unvegetated beach ridges, the widespread occurrence of which first attracted the attention of the Geological Survey (Wilkinson, 1900, 1907). Although discussed by Ting (1936, 1937), the most detailed studies of these raised coastal features are by McCann (1964, 1968) and Dawson (1979a, 1982).
In western Jura, raised coastal terraces can be traced almost continuously southward from Shian Bay as far as Inver (cf. Dawson, 1991, figure 5). Additional areas of raised coastal terrace occur at Corpach Bay
Analysis of the regional altitude variations of the highest raised beach terraces on the western Jura coast suggest the existence of two shorelines. The older of these (shoreline L1) declines in altitude to the south-west, from 40 m OD at Corpach Bay to 34 m OD at Bigh Gleann Righ Mor, 1.5 km east of Ruantallain. This raised shoreline is also thought to occur in northern Islay; it has a regional gradient of 0.56 m km−1 (Dawson, 1982). A separate and slightly younger shoreline (L2) is considered to be present in south-west Jura. This shoreline declines in altitude to the south-west, from 31 m OD at Glenbatrick to 24 m OD at Inver and has a regional gradient of 0.53 m km−1. It was therefore inferred by Dawson (1979a, 1982) that south-west Jura remained ice-covered while shoreline L1 was formed between Corpach Bay and Shian Bay, and also on the northern Islay coastline. Dawson (1979a, 1982) also concluded that, owing to the drop in the marine limit between Corpach Bay and Glendebadel, north-west Jura was also ice-covered during this period. Deglaciation of south-west Jura took place at a slightly later date and was accompanied by the formation of shoreline L2.
The pattern of ridge-crest altitudes exhibited in the western Jura shingle 'staircases' indicates that, although stillstands may have occurred during the fall in the sea level from the marine limit, there were no major sea-level oscillations as relative sea level fell from near 35 m to 20 m OD. Most of the western Jura shingle spreads below this altitude terminate at the cliffiine of the Main Rock Platform and consequently patterns of sea-level change lower than 20 m OD cannot be established. One exception, however, occurs at South Shian Bay, where owing to the exceptionally low altitude of the High Rock Platform, raised beach gravels descend to almost 11 m OD (
The presence on the west coast of Jura of extensive spreads of Late Devensian raised shingle is primarily due to the glacio-isostatic uplift of shoreline Ll and its altitudinal relationship with the till-covered High Rock Platform. Thus upon deglaciation, the maximum level of the sea along this stretch of coast (34 40 m OD) stood several metres higher than the inner edge of the High Rock Platform. Marine erosion of the till cover during the ensuing fall in relative sea level resulted in extensive shingle deposition. This process would have undoubtedly been promoted by the gentle seaward slope of the underlying platform surface and also by the exposure of the coastal zone to the effects of Atlantic waves.
On the western side of the Paps of Jura, there occurs perhaps the finest example of a fossil medial moraine in the British Isles (Dawson, 1979b). It is 3.5 km long and trends approximately NW–SE. It originates at 450 m OD at the western foot of Beinn an Oir
The boulder complex is composed in places of up to four parallel lines of angular blocks, each line rarely exceeding 27 m in width and 2.5 m in vertical thickness. The boulders in the belts, almost entirely of Dalradian quartzite (the local bedrock) though occasionally of slate and phyl-lite, bear no evidence of striation or ice moulding. They range from 0.2 m to 1.3 m in length and contrast markedly with the generally smaller quartzite blocks that are found in local till exposures. Additionally, the mean diameter of boulders measured at 500 m intervals along the feature decreases seaward by 0.07 m km−1.
For most of its length the junction between each belt and the vegetation cover exhibits little variation in relief, though in places it is characterized by small boulder 'cliffs' up to 2 m in height. The main boulder belts are oriented parallel to each other; they are separate units which rarely merge. Coalescence of the belts is limited to the crest and flanks of Cnoc na Sgrioba, where the entire orientation of the feature changes slightly.
Dawson (1979b) argued that the medial moraine was produced during the waning of the Late Devensian ice-sheet. He suggested that the boulders of the moraine were deposited supraglacially from the Beinn an Oir nunatak on to a relatively thin, yet dynamically active, ice mass. Its preservation in the landscape as a series of unvegetated quartzite boulder belts is remarkable. Other medial moraines have been described from former Loch Lomond Readvance glaciers (see for example, Beinn Alligin). Only one similar feature, near Strollamus on Skye (Ballantyne, 1988; Benn, 1991), has been ascribed to the Late Devensian ice-sheet. No other medial moraines in Scotland compare with Sgriob na Caillich in either size or complexity.
Throughout western Jura, most Holocene beach accumulations mantle the rock surfaces of the Main and Low Rock Platforms. Relatively few raised coastal terraces are present and those that occur exhibit a gradual decline in altitude on Jura from near 10 m in the north-west to 8.5 m in the south-west. Most coastal areas, however, are characterized by banks of shingle and by shingle ridge 'staircases'. The most spectacular suite of Holocene shingle ridges occurs north of Inver, south-west Jura, where 31 unvegetated raised beach ridges descend from 12.3 m OD to the modern beach (Dawson, 1979a, 1991).
The highest ridge appears to have been produced during the culmination of the Main Postglacial medial moraine (centre) descends to the level of a high Transgression. Hence the staircase of 31 ridges and intervening swales are likely to have been produced during the last 6000–7000 years, largely as a result of decreasing rates of glacio-isostatic uplift during this period.
The west coast of Jura is therefore outstanding for its assemblage of raised coastal landforms and deposits. Both the range of features and their extent and degree of development are exceptional. The interest includes not only examples of the three major rock platforms recognized in western Scotland, the High, Main and Low Rock Platforms, but also spreads of unvegetated Lateglacial and Holocene shingle beach ridges unparalleled elsewhere in Scotland for the length of their morphological record of sea-level changes. The latter features, in particular, distinguish the west coast of Jura from northern Islay (see above). Elsewhere in Scotland, there are notable sequences of raised shingle ridges at Spey Bay and Tarbat Ness on the Moray Firth coast (Ogilvie, 1923). Those at Spey Bay are comparable in their scale of development to the Jura features but occur in a different geomorphological process environment, being associated with a major river (the Spey) and significant longshore drift. Moreover, they have not been studied in comparable detail to the features on the west coast of Jura.
The coastline of western Jura is one of the classic localities in Britain for raised beaches (formed by isostatic uplift — see Northern Islay above). It is characterized by a variety of well-developed coastal landforms, of which the spectacular, unvegetated spreads of raised beach shingle (formed during the last 14,500 years) are without parallel in Britain and have allowed a detailed pattern of relative sea-level changes to be reconstructed. The area also includes excellent examples of raised platforms cut in bedrock as well as the finest example of a medial moraine in Britain, a ridge of boulders deposited by the last ice-sheet (approximately 15,000 years ago).