Letters to the Editors

A FOSSIL ROCK GLACIER IN WESTER ROSS

SIRS, Fossil rock ghciers have not been described as occurring in Scotland (the present climate does not permit active ones). It is believed that an example exists near Loch Torridon in Wester Ross.

The feature begins at the foot of Beinn Alligin in a corrie-like valley head whose rock walls rise very steeply and, finally, precipitously for 500 m or more to culminate at 985 m (Fig. 1). It is composed of pieces of Torridon Sandstone (the bedrock of the area), few of those that can be seen being below boulder size: much of the visible debris is massive blocks up to 5 m or more long. Between the boulders cavities many metres deep penetrate into the interior of the feature and make it extremely difficult to traverse. The debris forms a continuous mass that rises to a maximal height of 12-15 m, is i-2 km long, 400 m wide near its source and tapers gradually to about 200 m near its terminus. It rests on a fairly gentle slope that falls from about 400 m altitude to about 260 m. The edge of the tongue-like mass of material is sharply defined. The surface relief (other than that provided by the blocks themselves) consists of ridges and depres­sions, some of the latter being closed. In the NW. half of the feature there are short ill-defined transverse ridges and clearer longitudinal ones. Down slope of this area for some 200 m is a system of well marked transverse ridges. Here the principal ridges have steep fronts on their down valley side, with many blocks dipping steeply (some almost vertically) down valley, giving the impression that the debris has been pushed along and slipped down in front of a huge bulldozer. The final 3-400 m of the debris mass is much lower than the remainder and comprises ill-defined ridges that tend to be transverse along with shallow depressions.

The debris was clearly provided by a large landslide. One side of the landslide scar is a vertical rock wall that descends from close to the highest summit of Beinn Alligin along the straight line of a major fault. The other side is partly a rock wall, controlled by joint planes and/or minor faults. Outside the landslide scar and the landslide track beneath it, the mountain slopes display the numerous steps corresponding with individual beds of Torridon Sandstone that typify corrie walls in this rock throughout NW. Scotland. In the area affected by the landslide, however, the steps are much fainter and often missing, implying that glacial plucking and severe periglacial frost riving have not affected the area since the landslide occurred. In the landslide track the rock steps have apparently been removed by the passage of the landslide debris. In part of the track, well up the mountain slope, the rock surface has the appearance of having been glacially smoothed: such an explanation is inapplicable in this location and it is inferred that the rock smoothing was caused by the landslide.

Scott. J. Geol. 11, (1), 83-86,1975

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84 LETTERS TO THE EDITORS

FIG. I . Fossil rock glacier by Beinn Alligin. i. Landslide scar and track. 2. Rock glacier: lines indicate ridges. 3. Landslide debris not incorporated in rock glacier. 4. Lateral moraines marking the limit of Loch Lomond Readvance. 5. Medial moraine. 6. Fluted moraines.

7. Hummocky moraines. Contour interval 100 m.

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LETTERS TO THE EDITORS 85

Since a large landslide has descended the mountain slope behind the fossil feature under consideration it may seem that the latter is nothing more than the debris of the landslide. This explanation is rejected. The writer has examined scores of former landslides in the field and/or on aerial photographs, including many in Torridon Sand­stone, and in no instance does the landslide material have a form in any way resembling the accumulation by Beinn Alligin. Where such landslides reached low ground the coarse blocky debris is banked up against the hill side. Furthermore, the edges of the debris mass resulting from such landslides are usually ragged, some blocks having travelled farther than the main mass: the Beinn Alligin feature has a sharply defined edge. The distance of 1*2 km that the debris extends from the mountain foot down a relatively gentle slope appears excessive for a landslide of the dimensions involved. Since at least the visible debris is angular and very coarse and the sandstone is not a suitable source for significant quantities of silt or clay, flowage' of the debris at the time of the landslide, or subsequently as a mudflow, appear improbable and cannot be reasonably invoked to account for the form of the feature or the distance it extends from the moun­tain foot.

The general and detailed form of the debris mass are remarkably similar to active and fossil rock glaciers in the Rockies and elsewhere. Active rock glaciers consist of a large mass of angular rock debris that is slowly transported by buried ice or by inter­stitial ice beneath the surface. They are typically a few hundred metres to 2-3 km long, are often lobate or tongue-like in plan, have sharply-defined margins, and rise (with buried ice) a few tens of metres above their surroundings. Transverse ridges, often convex down valley are characteristic, especially towards the terminus, and the surface may contain closed depressions. Rock glaciers are most common on corrie floors, where abundant debris is available from adjacent slopes, and they often develop when the decaying remnants of ice glaciers become covered with rock debris (Wahrhaftig and Cox 1959; White 1971). Because of these similarities and the apparent inadequacy of other explanations, it is suggested that the feature by Beinn Alligin is a fossil rock glacier.

The valley head in which the fossil feature lies was the source of a glacier that merged with ice from adjacent corries and from more distant sources when glacier ice last occupied the area. The maximal extent of these confluent glaciers is accurately defined by excellent lateral moraines that extend for several kilometers. Since such clear mor­aines occur in Scotland only in association with the Loch Lomond Readvance, it may be inferred that the fossil feature was formed after the maximum of this readvance, i.e. after c. 10 300 B.P.

If the landslide that provided the debris for the rock glacier occurred after the (ice) glaciers had completely disappeared it would be necessary to postulate the growth of ice within the debris mass. However, the evidence from pollen studies in Scotland as a whole points to conditions wholly unfavourable for such postglacial ice growth, especi­ally considering the relatively low altitude of c. 400-260 m of the fossil feature and its location only a few kilometers from the Scottish west coast.

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86 LETTERS TO THE EDITORS

On the other hand, it appears that extensive glacier ice did not exist when the land­slide occurred, for a mass of the landslide debris rests on the steep lower part of the mountain slope (Fig. i). (This mass of debris, terminating on its western side along a line perpendicular to the slope, is anomalous in relation to landsliding alone. On the interpretation proposed here, however, it can be explained as not having been incorpor­ated in the rock glacier.)

It is therefore suggested that the landslide occurred when the decaying glacier covered a very small area. It is possible that the presence of this glacier ice helped the landslide debris to travel farther from the mountain foot than it would otherwise have done, although this alone does not appear sufficient to account for the form of the fossil feature, especially its sharply defined margin. Rather is it suggested that the glacier remnant was temporarily reactivated by the weight of the landslide debris that buried it, a rock glacier thus being produced.

It may appear that rather special circumstances have been invoked to account for the development of the rock glacier. Apart from the available evidence apparently requiring such circumstances, it is also relevant that fossil rock glaciers have not been identified anywhere else in Scotland. While they may yet be found, the writer can state from his own fieldwork that no such feature exists in a large area of the Scottish Highlands despite the availability of numerous suitable locations (especially corries) at much higher altitudes than the fossil feature by Beinn Alligin. This apparent absence necessitates a fortuitous combination of circumstances to account for what may be a unique feature in Scotland.

ACKNOWLEDGEMENT

The writer is grateful to D. G. Sutherland for his comments on the draft of this paper.

REFERENCES

WAHRHAFTIG, C, and COX, A. 1959. Rock glaciers in the Alaska Range. Bullgeol Soc. Am. 70, 383-436.

WHITE, S. E. 1971. Rock glacier studies in the Colorado Front Range, 1961 to 1968. Arctic Alpine Res. 3, 43-64.

MS received 22nd May 1974 J. B. SISSONS

Department of Geography University of Edinburgh

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