Coire a'Cheud-chnoic

J.E. Gordon and D. G. Sutherland

Highlights

This outstanding geomorphological locality is noted for its assemblage of hummocky moraines formed during the Loch Lomond Stadial. These deposits provide important evidence for the processes of glacier activity and wastage.

Introduction

Coire a'Cheud-chnoic (Valley of a Hundred Hills) [NG 955 550] in Glen Torridon, is a classic geomorphological locality displaying one of the best and most accessible examples of hummocky moraine in Scotland. The moraines were first mentioned by Geikie (1863a) in a footnote (p. 157) reporting 'a vast but unrecorded accumulation of glacier mounds blocking up a glen between Loch Torridon and Loch Maree', and a plate of the valley, first published in the Geological Survey memoir (Peach et al., 1913b), has been reproduced in standard texts such as Wright (1937) and Sissons (1967a). Despite the clarity of development of the moraines little research was carried out on the area until recent years when Robinson (1977) mapped their distribution and Hodgson (1982, 1987) carried out sedimentological and morphological studies to ascertain the genesis of the features.

Description

Coire a'Cheud-chnoic drains into Glen Torridon from the south-east and is underlain by Torridonian sandstone and north-east aligned belts of Cambrian Quartzite. The greater part of the valley floor is covered by hummocky moraines which reach 8 m in height. Areas of bedrock crop out, implying that there is no great thickness of till under the moraines. Although they have a chaotic appearance, mapping of the moraines by Hodgson (1982, 1987) has shown that, in the lower valley, the majority of the moraines are elongated in a roughly north–south direction (in conformity with striations on the neighbouring bedrock surfaces) (Figure 6.13). Many of the moraines are asymmetric in long profile, with the highest part at the northern end, which gives them a conical appearance when viewed from Glen Torridon (Figure 6.14).

Constituent materials of the hummocky moraine are revealed in a recent roadside quarry at [NG 954 567]. The section shows till at the bottom, consisting of angular clasts and boulders in a sandy and gritty matrix. Its upper surface is deformed, and an overturned fold partly encloses a large lense of poorly bedded sands and gravels. Above, layers of sands and gravels form the core of the upper section of the mound. The depression adjacent to it is underlain by a semicircular lens of angular clasts in a gravel and grit matrix, and is capped by a layer of poorly sorted sands and gravels. It is not possible to assess how representative this section is of the hummocky moraine as a whole, although Hodgson (1982, 1987) showed that the moraines elsewhere are composed of till and that there is little vertical variation in either particle size or erratic content. He found that the majority of the moraine mounds were elongated in a north–south direction and that the clast fabrics had a primary orientation mode parallel to the long axis of the moraines. Although quartzite underlies all the steep ground above the valley, it represents only a small proportion of the surface boulders on the moraines, implying that supraglacially derived debris from the valley sides was not a significant source of the debris. Instead, a significant portion of the erratic content of the moraines is material that was introduced into the area during the Late Devensian ice-sheet glaciation. The mapping of Robinson (1977, 1987a) and Sissons (1977a) has demonstrated that the moraines in Coire a'Cheud-chnoic were formed during the Loch Lomond Readvance.

Interpretation

Hodgson (1982, 1987) concluded that, since the moraines were dominantly composed of exotic material but had been formed by glaciers that were confined to the local valley system, then the moraines were the result of a Loch Lomond Readvance glacier overriding and partially deforming pre-existing debris into a crude type of fluted moraine. He considered that such an explanation could be applied more widely to hummocky moraine in the Highlands of Scotland, although he cited only one particular study, that of Donner and West (1955) on Skye (see the Cuillin), which apparently described similar moraines. Detailed studies elsewhere in Torridon supported his contention that the fluted moraine had formed by subglacial deformation of the sediments (Hodgson, 1982, 1986).

Hummocky moraine occurs widely in the Highlands, Inner Hebrides and parts of the Southern Uplands (see, for example, Geikie, 1863a; Geikie, 1901; Bailey et al., 1924; Peacock, 1967; Sissons, 1967a, 1977a; Gray and Brooks, 1972; Sissons et a/., 1973; Bennett, 1991), but its best development in the British Isles is in northwest Scotland (Sissons, 1976b). The Torridon area contains some particularly notable examples, especially to the north of Beinn Eighe and Liathach (Sissons, 1977a; Hodgson, 1982). The moraines of Coire a'Cheud-chnoic are especially significant, both as a representative example of these landforms in the Torridon area and as an area on which detailed research has been carried out into their mode of formation. The locality is also well-known because of its use as a text-book illustration.

Despite considerable documentation of the location of hummocky moraine and some controversy over its implications for the reconstruction of former ice margins (see the Cairngorms), there have been few detailed studies of its origin until recently. Three main explanations have been proposed (see also Loch Skene and the Cuillin). First, it may be a type of chaotic, 'dead-ice' topography formed by rapid stagnation of ice that carried an extensive cover of supraglacial debris (Sissons, 1967a; Thompson, 1972). Second, it may be produced by controlled or uncontrolled deposition by actively retreating glaciers (Eyles, 1979, 1983; Day, 1983; Horsfield, 1983; Benn, 1990, 1991; Bennett, 1990, 1991; Bennett and Glasser, 1991) (see the Cairngorms). Third, as demonstrated by Hodgson (1982, 1987) for the Coire a'Cheud-chnoic features, it may be a subglacial deposit formed by deformation of preexisting till (see also the Cuillin; Ballantyne, 1989a, Benn, 1991). Probably all three types exist, often in a single area (as in the Cuillin and at Loch Skene), but their relative importance is generally unknown. As the focus of a detailed case study, Coire a'Cheud-chnoic is therefore an important reference locality for wider comparative investigations of the genesis of the different landform types that appear to comprise hummocky moraine. The results of such work may have a significant bearing on interpreting styles of deglaciation and recognition of diagnostic landform and sediment facies assemblages (see Eyles, 1979, 1983; Sharp, 1985; Evans, 1989; Benn, 1990, 1991; Benn et al., 1992; Bennett, 1991). The distribution patterns of hummocky moraine types, when compared with reconstructed glacier margins, flowlines and debris sources, should provide important information on sediment transfer controls and glacier processes during the Loch Lomond Readvance (cf. Benn, 1989b for end moraines). Further, if the subglacial origin of the Coire a'Cheud-chnoic deposits is confirmed, then the site can provide important field evidence for inputs to models of glacier dynamics for deformable bed materials (see Boulton and Jones, 1979; Alley et al., 1986; Boulton and Hindmarsh, 1987).

Conclusion

Coire a'Cheud-chnoic is a classic geomorphological locality for an assemblage of hummocky moraines formed by a glacier during the Loch Lomond Stadial (about 11,000–10,000 years ago). It is one of the best locations in Scotland showing this type of glacial deposit, the form and distribution of which are important for interpreting the mode and pattern of ice wastage.

References