Glacial activity and paraglacial landsliding in the Devensian Lateglacial: evidence from Craig Cerrig-gleisiad and Fan Dringarth, Fforest Fawr (Brecon Beacons), South Wales

Abstract

The tongue-shaped mass of debris and associated ridges on the cirque floor below Craig Cerrig-gleisiad. BreconBeacons National Park is important and controversial because it has been attributed to more than one glacier advance during the Late Devensian. A new origin is proposed involving landslide development from the collapse of part of the western headwall followed by a single phase of glacier development in the Loch Lomond Stadial (Younger Dryas), which reworked the landslide sediments. Evidence for this landslide. which provides useful criteria for differentiating moraines formed by small glaciers from landslides, lies in tension cracks, backward-tilted blocks and bedrock joints dipping out of the western headwall, together with lateral levkes, upstanding termini and angular clasts with only occasional, indistinct striae on the tongue-shaped mass. which is interpreted as a flowslide. Glacier reworking of debris in the upper part of the Cwm Cerrig-gleisiad landslide is indicated by subparallel ridges rising to 20 m above the cirque floor containing abraded clasts (16-32% striated). This interpretation is supported by a comparison with the morphological and sedimentary characteristics of a neighbouring landslide at Fan Dringarth. where no glacier developed in the Loch Lomond Stadial. The existence of paraglacial landsliding has significant palaeoenvironmental implications leading to: ( I ) erroneously large estimates of equilibrium line depression (AELA) in the Loch Lomond Stadial; (2) consequent underestimates of summer palaeotemperatures and/or overestimates of the contribution of wind-drifted snow to glacier accumulation; and (3) larger moraines than usual and overestimation of the efficacy of glacial erosion because of antecedent processes.Lateglacial. The Fan Dringarth landslide is regarded as analogous to some of the landforms below Craig Cerrig-gleisiad without the complication of glacier development. Evidence from both sites is used as a basis for considering the palaeoenvironniental implications, including Lateglacial glacier development in the Rrecon Beacons, the contribution of glacial activity to bedrock excavation and the contribution of paraglacial landslide processes to scarp retreat and erosion rates at this time. CWM CEKKIG-GLEISIAD: EROSIONAL AND DEPOSITIONAL LANDFORM COMPLEXNorth of Craig Cerrig-gleisiad ( Figure I ) is a cirque hollow (Cwm Cerrig-gleisiad) which has been cut southwards and westwards mainly into south-east dipping (mean bearing, 155'; mean dip, 15"; for eight measurements from the headwall zone) Brownstones and underlying Senni Beds of the Old Red Sandstone (ORS) (Barclay et (11. 1988). The landforms can be subdivided into two headwall sections and four groups of depositional landforms (Figures 2 and 3 ) . Soictlwr-n hctitinwffThe c. 500 m long southern section of the headwall is aligned approximately east west and rises about 200 m above the cirque floor. I t comprises steep exposed bedrock faces deeply cut by gullies in its upper part and mostly stable, vegetated talus and ...

Exposure dating and reinterpretation of coarse debris accumulations (‘rock glaciers’) in the Cairngorm Mountains, Scotland

“…In the mountains of Great Britain, a small number of rock-slope failures are known to have occurred during the Late Devensian Lateglacial, during the interval between ice-sheet deglaciation and the beginning of the Holocene (e.g. Ballantyne, 1986b;Shakesby and Matthews, 1996), but surface exposure dating using cosmogenic isotopes has shown that other major landslides occurred much later. A major rockslide at The Storr on the Isle of Skye occurred at 6.5 AE 0.5 ka (Ballantyne et al, 1998), a massive rock avalanche on Beinn Alligin in the NW Highlands has been dated to 4.0 AE 0.3 ka (Ballantyne and Stone, 2004) and a rock avalanche near Glencoe has been dated to ca.…”

Late Quaternary glaciation in Southern Africa: moraine ridges and glacial deposits at Mount Enterprise in the Drakensberg of the Eastern Cape Province, South Africa

“…It therefore is unlikely that ridges A and B are of protalus origin, although Shakesby (1997) has suggested that the rampart-talus distances reported by Ballantyne and Kirkbride (1986) may not be diagnostic of rampart origins. The arcuate plan of ridges A and B is unlikely to have originated from landsliding because there is no evidence of backward-tilting blocks or of tension cracks adjacent to the backwall, nor of lateral ridges and an upstanding terminus on the bench at 2000 m, all of which are diagnostic of landslides (Shakesby and Matthews, 1996). Shakesby (1992) considers that rock avalanches result in 'splays of debris forming ridges and mounds of debris'.…”

“…Some of the clasts within the deposits that form ridges A and B, where A is superimposed on B (Fig. 3), are striated, which strongly indicates a glacial origin (Boulton, 1978;Shakesby and Matthews, 1996). The particle size composition of the fine matrix of the sediments at the same site (Table 1) the particle-size distribution envelopes characteristic of lodgement and of melt-out tills (Sladen and Wrigley, 1983), although these envelopes are broad and therefore may not be diagnostic.…”

The glacial geomorphology of the Loch Lomond Stadial in Britain: a map and geographic information system resource of published evidence

“…Similarly, even when moraines have been inferred to pre-date the LLS, usually based on their more subdued appearance and greater size, the lack of absolute dates on these landforms means a LLS age cannot be ruled out. The glacial origin of some features has also been questioned, especially where moraines may appear similar in morphology to protalus ramparts or landslide blocks (Shakesby & Matthews, 1996). In cases of both uncertain age and genesis, this project assumes a LLS origin, especially if location appears consistent with the maximum extent of LLS glaciation, unless subsequent literature strongly refuted such an interpretation.…”