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. The original source material for these web pages has been made available by the JNCC under the Open Government Licence 3.0. Full details in the JNCC Open Data Policy
Eas na Broige Debris Cone
A. Werritty and L.J. McEwen
Highlights
The deposits in the debris cone at Eas na Broige provide an important geomorphological and sedimentary record of slope processes during the Holocene. They show successive phases of debris-flow activity and alluvial fan development.
Introduction
The Eas na Broige debris cone
Description
The local bedrock comprises granite on the lower slopes with rhyolite lavas on the higher ground; the Dalness Chasm having been etched from a porphyritic dyke (Bailey and Maufe, 1916). During the Loch Lomond Stadial, glacier ice extended up to 650 m OD in upper Glen Etive (Thorp, 1981a, 1986). The debris cone is thus Holocene in age.
The Eas na Broige cone comprises two units: an upper debris cone with a concave long profile and mean gradient of 13.7°, and a lower alluvial fan with a mean gradient of 6.2°. Five discrete cone and fan surfaces can be identified within the lower alluvial surfaces which are inset into the steeper debris cone surfaces. The respective volumes of the debris cone and alluvial fan have been estimated at 170,000 m3 and 100 m3; the latter being entirely derived by incision and reworking of the former (Brazier, 1987).
The stratigraphy of much of the cone has been exposed by stream incision, and at two contrasting sites clearly distinguishable debris flow and fluvial sediments have been identified
Interpretation
Three major phases in the development of this fluvially modified debris cone can be identified. The debris cone initially developed during the first 6000 years of the Holocene, with aggradation ceasing about 4000 BP as a result of exhaustion of the sediment supply through the Dalness Chasm. A prolonged period of stability then ensued until 550 BP. This was followed by a final phase in which the incision into the debris cone produced the inset alluvial fan. The pollen evidence strongly suggests that fluvial activity was contemporaneous with changes in the vegetation cover caused by human interference. The removal of the tree cover destabilized the cone surface and triggered fluvial incision. This instability has continued until the present day.
This site has a potentially wider significance in terms of it being representative of the class of fluvially modified debris cones found throughout the Highlands (Brazier et al., 1988) and other parts of upland Britain (Harvey et al., 1981). First, the initial accumulation of debris-flow deposits involved the reworking of sediment deposited during deglaciation. This implies that this cone, like many others in upland Britain, is 'paraglaciar in origin (cf. Ryder, 1971; Church and Ryder, 1972), that is, its formation was dependent upon an abundant sediment supply following deglaciation. Once this was exhausted, aggradation on the cone ceased. Second, fluvial incision at this site, is attributed to recent human disturbance of the vegetation. There are many other fluvially modified debris cones in the Scottish Highlands and upland Britain where a similar anthropogenic trigger may have initiated the same change in the process regime on debris cones (see Statham, 1976b; Harvey et al., 1981; Innes, 1983b). In this latter respect the Eas na Broige cone contrasts with those in Glen Feshie, where natural processes are considered to have been responsible for reactivation (Brazier and Ballantyne, 1989).
The Eas na Broige cone is a very good example of a Holocene debris cone that has been subject to fluvial modification. Although debris cones are ubiquitous throughout the Scottish Highlands, this site in Glen Etive is unique in that the date and extent of the fluvial reworking of the original cone have been precisely determined.
The Eas na Broige debris cone provides the most detailed record currently available of Holocene sedimentation at the margins of a major valley in Scotland. Following deglaciation at approximately 10,000 BP the debris cone developed over the next 4000 years. At this time the cone surface became stabilized until about 500 years ago, when fluvial reworking of the basal part of the cone arose in response to the removal of the forest cover by human activity. The cone thus represents a particularly good example of a fluvially modified debris cone in which the most recent phase of development has been in response to human settlement on the valley floor.
Conclusion
Eas na Broige debris cone is a fan-shaped accumulation of poorly sorted material (mixed particles of various sizes) formed by flows, from the slopes above, of rock and soil debris mixed with water. It provides an important record of slope processes during the Holocene and is representative of a type of landform and process system that occurs widely in the Highlands. In particular it shows two phases of development, the first reflecting high sediment supply following deglaciation (ice melting and retreat) and the second, the impact of forest clearance by Man. The site is not only a good landform example but has a well-documented history of development.