Colin K. Ballantyne

The periglaciation of Great Britain

Part I. Introduction and Context: 1. Introduction 2. Quaternary environmental change in Great Britain 3. Periglacial environments Part II. The Periglaciation of Lowland Britain: 4. Ice wedge casts and relict tundra polygons 5. Ground ice depressions and related phenomena 6. Active layer processes: cryoturbation and patterned ground 7. Periglacial mass-wasting and slope evolution in lowland Britain 8. Lowland landscape modification by fluvial and aeolian processes Part III. The Periglaciation of Up-Land Britain: 9. Frost weathering and mountain-top detritus 10. Patterned ground on British mountains 11. Solifluction landforms in upland Britain 12. Talus slopes and related landforms 13. Nival, fluvial, aeolian and coastal features Part VI. Periglacial Environments: 14. Past and present periglacial environments.

Reconstructing the transport history of glacigenic sediments: a new approach based on the co-variance of clast form indices

Abstract Two indices are used to describe the aggregate shape and roundness characteristics of glacially transported clasts: the C40 index (the percentage of clasts with c a ratios ⩽ 0.4 ) and the RA index (the percentage of very angular and angular clasts in a sample). Analysis of the between- and within-sample co-variance of these indices provides a powerful tool for discriminating actively- and passively-transported clasts in glacigenic deposits. Use of these indices is illustrated in an analysis of the form characteristics of clasts from lateral and frontal moraines at Storbreen, Jotunheimen. These fall between those of actively transported clasts in basal till (low C40 and RA values) and those of unmodified frost-weathered clasts (high C40 and RA values). Both indices decline down-moraine. Within-sample analyses of clast shape demonstrates that these aggregate clast form gradients reflect changes in the relative proportions of actively and passively transported debris on the moraines rather than progressive modification of actively transported clasts. These analyses also indicate that actively transported clasts form the dominant component of the moraines, particularly near the former glacier terminus. Further applications of the approach developed here are suggested.

Exposure dating and validation of periglacial weathering limits, northwest Scotland

Periglacial weathering limits on two mountains in northwest Scotland separate zones of contrasting exposure history. Exposure dating of bedrock below the weathering limits gives ages consistent with late Devensian deglaciation, but six out of seven samples from above the weathering limits give minimum exposure ages older than late Devensian ice expansion. These results suggest that mountain summits stood as nunataks above the last ice-sheet surface and rule out formation of weathering limits by periglacial rock breakdown since deglaciation, trimming of frost debris during an ice-sheet readvance, or enhanced weathering prior to climatic warming during ice-sheet downwastage. The dating results do not preclude the possibility that weathering limits mark a former englacial boundary between passive cold-based ice on mountain summits and erosive, warm-based ice at lower elevations, although 26 Al/ 10 Be ratios for high-level bedrock surfaces provide no evidence of prolonged static ice cover.

Paraglacial Slope Adjustment and Resedimentation Following Recent Glacier Retreat, Fabergstolsdalen, Norway

Withdrawal of glacier ice from upper Fabergstolsdalen after A.D. 1930 has exposed steep drift-covered slopes on the north side of the valley. Since 1943 these have experienced radical transformatio...

Periglacial trimlines in the Scottish Highlands

Abstract Two periglacial weathering limits have been identified on mountains in the Highlands of Scotland. The lower has been interpreted as a periglacial trimline marking the upper limits of Loch Lomond (Younger Dryas) Stade glaciers, and has been employed to reconstruct the dimensions of parts of the main West Highland icefield and certain smaller glaciers and icefields. Interpretation of the upper weathering limit has been more contentious, but recent research has demonstrated: (1) significant contrasts in the degree of bedrock weathering above and below this limit; (2) that gibbsite, a pre-Devensian weathering product, is abundant above the limit but rare in soils farther downslope; (3) that the isotopic (26Al) age of bedrock exposures above the limit greatly exceeds that at lower altitudes; and (4) that the altitude of the weathering limit descends regularly along former ice-sheet flowlines. Collectively, these attributes suggest that the upper weathering limit is a periglacial trimline defining the maximum altitude of the last ice sheet. Calculated basal shear stress values are consistent with this interpretation, which implies that the mainland ice-shed lay at ca. 900 m in Wester Ross and descended gently northwards, whilst Harris and Lewis in the Outer Hebrides supported an independent ice-cap that achieved a maximum altitude of ca. 700 m. Wider implications of this interpretation and the future potential for ice-sheet reconstruction are explored.

The Beinn Alligin rock avalanche, NW Scotland: cosmogenic 10Be dating, interpretation and significance

A tongue of very coarse rockslide debris that extends 1.25km downvalley below Beinn Alligin in NW Scotland has been variously interpreted as a glacier-cored rock glacier, landslide debris redistributed by glacier ice or an excess-runout landslide. Exposure dating with cosmogenic 10Be demonstrates that the the debris mass was emplaced at 3950±320 yr BP, and therefore was not associated with glacier ice. Calculations based on frictional considerations imply that the feature is an excess-runout rock avalanche (sturzstrom) deposit. The morphological characteristics of the deposit appear consistent with movement by grainflow or fragmental flow. Failure is inferred to reflect time-dependent paraglacial stress release and consequent propagation of an internal joint network, but may have been triggered by seismic activity. The late-Holocene age of failure implies persistence of the eŒects of paraglacial stress release over a time-scale of several millennia.

Cosmogenic Cl-36 dating of postglacial landsliding at The Storr, Isle of Skye, Scotland

Major postglacial rock slope failures are a common feature of the Scottish Highlands and other mountainous areas that were deglaciated at the end of the Pleistocene, but evaluation of the causes and triggers of failure has been hindered by a lack of reliable dating evidence. We report the result of a pilot study designed to establish the absolute age of a large postglacial rotational rockslide at The Storr on the Isle of Skye, Scotland, using 36Cl surface exposure dating. Exposure ages of 6.3 ± 0.7 cal. ka BP and 6.6 ± 0.8 cal. ka BP were obtained for rock samples from two separate landslide blocks, giving an overall age estimate of 6.5 ± 0.5 cal. ka BP for rock slope failure at this site. This date is consistent with AMS radiocarbon dating of windblown sand derived from the failure scarp, and with previous inferences (based on relative dating evidence) concerning an early-Holocene age for most rock slope failures in the Scottish Highlands. The long time lag (. 7 ka) between deglaciation and failure suggests that progressive joint extension and shearing of rock bridges and asperities were of critical importance in conditioning failure, though a seismic trigger cannot be ruled out. The methodology of surface exposure dating in this context is described and its future potential assessed.

The Late Devensian periglaciation of upland Scotland

Abstract A wide variety of Late Devensian periglacial landforms developed on Scottish mountains both before ca. 13,000 BP and during the Loch Lomond Stadial of ca. 11,000-10,000 BP. Nearly all such features are now inactive. Late Devensian periglacial weathering produced three types of regolith mantle (openwork block deposits, sandy diamicts and silt-rich frost-susceptible diamicts), each of which supports a characteristic assemblage of relict landforms. On upper slopes these include large-scale sorted circles and stripes, earth hummocks and nonsorted relief stripes, sorted and nonsorted solifluction features, massive boulder sheets and lobes, and nivation benches. Talus, protalus ramparts, rock glaciers and alluvial fans also developed at the base of mountain slopes. The distribution of Late Devensian periglacial features on Scottish mountains is locally controlled by topography, the response of underlying rocks to periglacial weathering and the limits of former glaciers. Regional variations in the altitude of certain forms of Loch Lomond Stadial age (particularly protalus ramparts and rock glaciers) indicate a decrease in former snowfall eastwards across the Scottish Highlands and northwards from the Highland Boundary Fault. Several upland periglacial features are also diagnostic of former permafrost, and complement palaeotemperature reconstructions based on ice-wedge casts and the equilibrium firn line altitudes of stadial glaciers. These suggest that under stadial conditions mean January temperatures at 600 m and 1000 m on mountains in the Western Grampians must have been no higher than −20°C and −23°C respectively, and possibly several degrees lower.

Evidence for a glacial readvance pre-dating the Loch Lomond Advance in Wester Ross

Synopsis A series of lateral moraines and other ice-marginal features in the Wester Ross area of the NW Highlands is described. These total nearly 30 km in length, and indicate the former existence of a lobe of ice nearly 25 km in width across Lochs Gairloch and Ewe and the surrounding low ground. The ice-marginal features are interpreted as marking the limit of a glacial readvance that took place in this area sometime between the Late Devensian maximum of 17 000–18 000 B.P. and the onset of the Lateglacial Interstadial. The name Wester Ross Readvance is suggested.

Late Holocene debris cone evolution in Glen Feshie, western Cairngorm Mountains, Scotland

Recent river erosion of three coalescing debris cones in Glen Feshie has exposed a complex sequence of debris flow units. Radiocarbon dating of organic matter from interbedded buried soils reveals that the soil at the base of the sequence was buried at c. 2000 yr BP, but that the bulk of the cones accumulated since the fifteenth century AD. The episodic nature of cone development is attributable to lateral migration of the River Feshie, with periods of cone accumulation when the river occupied the far side of its floodplain alternating with periods of erosion when the river impinged on the cones. There is no evidence to suggest that recent cone accumulation is related to anthropogenic vegetation disturbance, but phases of cone accumulation show a broad temporal correspondence with periods of Late Holocene climatic deterioration. The cones are essentially paraglacial in that their continuing accumulation depends on a supply of sediment derived from glacial and periglacial deposits upslope. The form of debris-flow units indicates that flows at this site were less viscous than most ‘hillslope’ flows, and cone volumes indicate an average annual accumulation of c. 50–60 m 3 of sediment over the past c. 300years.