Late Quaternary alluvial fans, debris cones and talus cones in the Grampian Highlands, Scotland

Abstract

Alluvial fans, debris cones and rockfall talus cones are widespread in upland Britain, but remarkably little is known about their characteristics, development and significance. This research project has three main objectives: 1. to establish the morphological and surface sedimentary characteristics of alluvial fans, debris cones and talus cones in the Grampian Highlands of Scotland; 2. to identify the factors that have controlled their formation and distribution; and 3. to determine the timing, nature and rate of fan- and cone-forming processes. On the basis of previous literature, an Orioni model that describes a continuum of fan and cone morphological and surface sedimentary properties was devised. The applicability of this model was tested using data for six variables (long profile gradient, slope form, downslope changes in clast size, roundness and form, and a scale ratio of maximum clast size to total fan or cone length) obtained for fans and cones in the Grampian Highlands and the Lyngen Peninsula in Northern Norway. The results of these tests were then used to produce a modified model appropriate to fans and cones in upland Britain. Using a combination of map, field and aerial photograph data, several environmental and morphometric controls on the distribution and type of fan and cone development were investigated. The dimensions of different types of fan and cone are shown to be determined by basin morphometry, lithology and glacial history. Discriminant analysis identified basin gradient, basin width and basin height as the principal catchment properties that influence the dominant type of fan- or cone-forming process. Stratigraphic and radiocarbon evidence suggests that many debris cones are essentially paraglacial landforms that formed in the earlier part of the Flandrian. Many of these cones have subsequently been modified in the late Flandrian by fluvial processes, in some cases in response to anthropogenic interference. However, evidence from one site has also revealed that substantial debris cone aggradation has occurred since c 300 BP, implying high rates of denudation in the recent past at this site. The volumes of other debris cones imply that as much as 1-3m of surface lowering has occurred in gullies upslope since deglaciation. Much lower values of surface lowering are associated with alluvial fan development, suggesting that, locally at least, denudation by fluvial processes has been less significant than denudation resulting from debris flow.