Loch Lomond Stadial Glaciers Research Articles

Glacially moulded landslide runout debris in the Scottish Highlands

ABSTRACTA tongue of hummocky terrain ∼1 km long and ∼400 m wide extends downslope from the source area of a rock-slope failure that formed the summit arête of Sgùrr nan Ceathreamhnan in the NW Highlands. The tongue descends from ∼810 m to ∼650 m, crosses a corrie obliquely and laps onto an opposing slope. Individual hummocks are circular to elongate, up to 6 m high and streamlined. A possible origin as recessional or ice-stagnation moraines is inconsistent with hummock morphology and the alignment of the hummock belt, and the streamlining of the hummocks is incompatible with the form of unmodified rock-avalanche runout hummocks. It is proposed that the tongue of hummocky terrain represents rock-slope failure during or after ice-sheet deglaciation, and subsequent modification of runout debris by subglacial erosion during the Loch Lomond Stade (∼12.9–11.7 ka). This interpretation implies (i) that the debris was deposited by an excess-runout rock avalanche; (ii) that the glacier that subsequently occupied the corrie was warm-based; (iii) that Lateglacial landslide runout debris was not invariably evacuated by Loch Lomond Stadial glaciers, as previously suggested; and (iv) that some features interpreted as hummocky moraines elsewhere may have a similar origin.

Dynamics and palaeoclimatic significance of a Loch Lomond Stadial glacier: Coire Ardair, Creag Meagaidh, Western Highlands, Scotland

The extent, basal conditions and retreat history of a Loch Lomond Stadial glacier are reconstructed based on detailed geomorphological and sedimentological assessment. We present new evidence from the vicinity of Coire Ardair that supports the former existence of a warm-based, locally-fed valley glacier, with probable cold-based ice on the surrounding plateau. This is broadly consistent with modelled creep-dominated flow in the upper catchment and sliding-dominated flow throughout much of the valley. A dense suite of moraines, primarily formed in ice-marginal environments, records a multi-phase recessional history: (1) active and oscillatory retreat; (2) a prolonged ice stillstand; (3) partial ice stagnation with occasional minor readvances; (4) increased oscillatory retreat with a substantial readvance event; and (5) rapid and uninterrupted retreat. We propose that a Coire Ardair glacier responded to sub-centennial scale climate fluctuations, possibly associated with the periodic delivery of warmer air masses to the region, rather than to a single, prominent shift in climate.

Enhanced rock‐slope failure following ice‐sheet deglaciation: timing and causes

ABSTRACTThe temporal pattern of rock‐slope failures (RSFs) following Late Pleistocene deglaciation on tectonically stable terrains is controversial: previous studies variously suggest (1) a rapid response due to removal of supporting ice (‘debuttressing’), (2) a progressive decline in RSF frequency, and (3) a millennial‐scale delay before peak RSF activity. We test these competing models through beryllium‐10 (10Be) exposure dating of five closely‐spaced quartzite RSFs on the Isle of Jura, Scotland, to establish the relationship between timing of failure and those of deglaciation, episodes of rapid warming and periods of rapid glacio‐isostatic uplift. All five dated RSFs occurred at least 720–2240 years after deglaciation, with the probability of failure peaking ~2 ka after deglaciation, consistent with millennial‐scale delay model (3). This excludes debuttressing as an immediate cause of failure, though it is likely that time‐dependent stress release due to deglacial unloading resulted in progressive development of failure planes within the rock. Thaw of permafrost ice in joints is unlikely to have been a prime trigger of failure as some RSFs occurred several centuries after the onset of interstadial warming. Conversely, the timespan of the RSFs coincides with the period of maximum glacio‐isostatic crustal uplift, suggesting that failure was triggered by uplift‐driven seismic events acting on fractured rock masses. Implications of this and related research are: (1) that retreat of the last Pleistocene ice sheets across tectonically‐stable mountainous terrains was succeeded by a period of enhanced rock‐slope failure due to deglacial unloading and probably uplift‐driven seismicity; (2) that the great majority of RSFs in the British Isles outside the limits of Loch Lomond Stadial (= Younger Dryas) glaciation are of Lateglacial (pre‐Holocene) age; and (3) numerous RSFs must also have occurred inside Loch Lomond Stadial (LLS) glacial limits, but that runout debris was removed by LLS glaciers. Copyright © 2013 John Wiley & Sons, Ltd.

Chronology of glaciation and deglaciation during the Loch Lomond (Younger Dryas) Stade in the Scottish Highlands: implications of recalibrated 10Be exposure ages

Considerable uncertainty surrounds the timing of glacier advance and retreat during the Younger Dryas or Loch Lomond Stade (LLS) in the Scottish Highlands. Some studies favour ice advance until near the end of the stade (c. 11.7 ka), whereas others support the culmination of glacier advance in mid‐stade (c. 12.6–12.4 ka). Most published 10 Be exposure ages reported for boulders on moraines or deglacial sites post‐date the end of the LLS, and thus appear to favour the former view, but recalibration of 33 10 Be ages using a locally derived 10 Be production rate and assuming rock surface erosion rates of zero to 1 mm ka−1 produces exposure ages 130–980 years older than those originally reported. The recalibrated ages are filtered to exclude anomalous data, and then employed to generate aggregate probability density distributions for the timing of moraine deposition and deglaciation. The results suggest that the most probable age for the timing of the deposition of the sampled outermost moraines lies in the interval 12.4–12.1 ka or earlier. Deglacial ages obtained for sites inside Loch Lomond Stadial glacier limits imply that glaciers at some or all of the sampled sites were retreating prior to 12.1 ka. Use of aggregated data does not exclude the possibility of asynchronous glacier behaviour at different sites, but confirms that some glaciers reached their maximum limits and began to retreat several centuries before the rapid warming that terminated the LLS at 11.7–11.6 ka, consistent with the retrodictions of recent numerical modelling experiments and with geomorphological evidence for gradual oscillatory ice‐margin retreat under stadial conditions.

Loch Lomond Stadial glaciers in North Harris, Outer Hebrides, North-West Scotland: glacier reconstruction and palaeoclimatic implications

Geomorphological mapping of North Harris provides evidence for the former existence of 10 glaciers with a total area of ca 35km2. A Loch Lomond (Younger Dryas) Stadial age (ca 12.9–11.5kyrBP) for this glacial episode is inferred from glacier configuration, landsystems dominated by hummocky recessional moraines, and relationships with Lateglacial periglacial phenomena. Equilibrium line altitudes (ELAs) of 150–289m were calculated for individual glaciers. ELA variability mainly reflects differences in snow-contributing area. The area-weighted mean ELA (204m) is consistent with a northwards decline in ELAs along the western seaboard of the British Isles of 69.5m (100km)−1, equivalent to a northwards ablation-season temperature decrease of 0.42°C (100km)−1. This latitudinal temperature gradient implies a mean July sea-level temperature of ca 7.2°C for the coldest part of the stade, roughly 6°C lower than at present. Sea-level precipitation at the time of the LLS glacial maximum is inferred to have been between ca 1970±200 and 2350±200mmyr−1, implying that LLS precipitation was up to 25% greater than now. Patterns of recessional moraines indicate that the glaciers remained close to climatic equilibrium as they retreated to their sources, though moraine belts implying near-stationary or readvancing ice margins on flat valley floors are separated by moraine-free zones indicating uninterrupted retreat. Calculation of ELAs for ‘residual’ glaciers in former source areas suggests that summer warming of ⩽1.0°C would have resulted in shrinkage of the glaciers to their sources.

Loch Lomond Stadial Glaciers in the Uig Hills, Western Lewis, Scotland

Abstract Geomorphological mapping of the Uig Hills provides evidence for four locally‐nourished valley glaciers with a total area of 11.69 km2. A Loch Lomond Stadial (∼12.9 — 11.5 ka BP) age for this glacial episode is indicated by the radial configuration of reconstructed glaciers, the landform assemblage and the absence of frost‐weathered rock inside its limits. Recessional moraines extend upvalley from former glacier termini, implying that glacier retreat was driven by gradual climatic forcing. The mean equilibrium line altitude (ELA) of the glaciers is calculated as 212 m, consistent with a northwards ELA decrease along the western seaboard of Great Britain of 68.5m/100 km, equivalent to a latitudinal summer temperature gradient of ∼0.41°C/100 km. A mean July sea‐level temperature of ∼7.1°C is inferred for the coldest part of the stade. Average accumulation at the ELA is estimated to have been 2585 ± 200 mm, much of which probably comprised snow blown from adjacent ground.

Loch Lomond Stadial Glaciers in the Uig Hills, Western Lewis, Scotland

Geomorphological mapping of the Uig Hills provides evidence for four locally‐nourished valley glaciers with a total area of 11.69 km2. A Loch Lomond Stadial (∼12.9 — 11.5 ka BP) age for this glacial episode is indicated by the radial configuration of reconstructed glaciers, the landform assemblage and the absence of frost‐weathered rock inside its limits. Recessional moraines extend upvalley from former glacier termini, implying that glacier retreat was driven by gradual climatic forcing. The mean equilibrium line altitude (ELA) of the glaciers is calculated as 212 m, consistent with a northwards ELA decrease along the western seaboard of Great Britain of 68.5m/100 km, equivalent to a latitudinal summer temperature gradient of ∼0.41°C/100 km. A mean July sea‐level temperature of ∼7.1°C is inferred for the coldest part of the stade. Average accumulation at the ELA is estimated to have been 2585 ± 200 mm, much of which probably comprised snow blown from adjacent ground.

Loch Lomond Stadial glaciers in the Aran and Arenig Mountains, North Wales, Great Britain

AbstractGeomorphological evidence for four former local glaciers has been mapped in the Aran and Arenig Mountains, North Wales. Former glacial extent was deduced from the distribution and assemblage of end and lateral moraines, hummocky moraine, boulder limits, drift limits and periglacial trimlines. Comparison of infilled lake sediment stratigraphies inside and outside of the former glacier limits suggests a Loch Lomond Stadial (Late Devensian) age of the former glaciers (c. 12.9–11.5 cal. ka BP). This finding is also supported by periglacial–landform contrasts between the land inside and outside of the glacier limits. Reconstruction of the four glaciers illustrates a mean equilibrium line altitude (ELA) of c. 504 m. From the reconstructed ELAs and the combination of precipitation and snowblow input for total accumulation, by analogy with Norwegian glaciers, a mean sea‐level July temperature is calculated at 8.4°C. Copyright © 2002 John Wiley & Sons, Ltd.

Loch Lomond Stadial glaciers in the Aran and Arenig Mountains, North Wales, Great Britain

Loch Lomond Stadial glaciers in the Aran and Arenig Mountains, North Wales, Great Britain

Further glacier and snowbed sites of inferred Loch Lomond Stadial age in the northern Lake District, England

Field mapping and evaluation of morphological and some sedimentological characteristics indicate that three debris accumulations in the hills centred on Knott in the northern Lake District result from glacial deposition. Another debris accumulation is interpreted as a pronival (protalus) rampart that developed at the downslope margin of a former snowbed. A Loch Lomond Stadial (LLS) age (11-10 k 14 C years bp ) is inferred for all the landforms and extends distribution of LLS glaciation into a new area of the Lake District. Reconstruction of the former glaciers reveals that two had larger surface areas than a substantial number of previously recognized LLS glaciers and estimated equilibrium line altitudes were lower than those for glaciers in the neighbouring Blencathra group hills. Glacier and snowbed development were favoured by site aspects, which provided protection from direct solar radiation, and occurrence of adjacent high ground, from which snow could be transferred by wind to contribute to glacier and snowbed mass. It is possible that other LLS glacier and snowbed sites in the Lake District remain to be discovered.

Empirical testing of Loch Lomond Stadial glacier limits derived from a glaciological model for the Applecross Peninsula, Wester Ross, Scotland

A high resolution glaciological model of Loch Lomond Stadial glaciers on the Applecross Peninsula is tested by geomorphological mapping. Results indicate strong similarities between limits derived from the model output and those found empirically. Discrepancies between the two are largely the function of model limitations but also poorly defined limits in the field. Such overall close matching highlights the value of an approach linking high resolution modelling with detailed field mapping. It can be used to gain important insights into glacier dynamics and the palaeo‐environment during the Loch Lomond Stadial.

Characteristics and implications of some Loch Lomond Stadial moraine ridges and later landforms, eastern Lake District, northern England

Moraine ridges and mounds of inferred Loch Lomond Stadial (LLS) age have been mapped at three sites (Fordingdale, Swindale and Wet Sleddale) in part of the eastern Lake District, northern England, and indicate glaciers were more widespread than envisaged by Sissons (1980, Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 71, pp. 13–27). The moraines delimit closely the downslope/downvalley limits of the former glaciers but there is no geomorphological evidence with which to define their upslope/upvalley margins. The former glaciers are considered to have been nourished within the confines of their individual valley, cirque and hillside embayment respectively, rather than being outlet glaciers of plateau icefields. Estimated equilibrium line altitudes (ELAs) are within the range of values determined previously for LLS glaciers in the Lake District and do not necessitate revision of established palaeoclimatic parameters. Individual ELAs were probably influenced by local factors; all three former glaciers had accumulation-area aspects between north and east, limiting the impact of direct solar radiation during the ablation season, and were adjacent to extensive areas of high ground to the west and/or south that would have facilitated transfer of snow to their surfaces by winds from those directions. In Fordingdale, three essentially contemporaneous depositional landforms occur upslope of the moraines and are considered to represent hillslope adjustments following wastage of glacier ice from the site. © 1998 John Wiley & Sons, Ltd.

Landforms associated with a Loch Lomond Stadial glacier at Cronkley Scar, Teesdale, northern Pennines

SUMMARY Based on its location, morphology and lithological composition the large depositional ridge (Tarn Rigg) at Cronkley Scar in upper Teesdale is interpreted as the end moraine of an escarpment glacier rather than a lateral moraine of a much larger valley glacier in Teesdale as previous researchers had thought. Ridge dimensions indicate that the glacier had stable margins for a considerable period of time and was probably in an equilibrium condition. Estimates of the equilibrium line altitude and July palaeotemperature suggest that significant inputs of wind-blown snow from adjacent high ground to the south-west were critical for glacier development at this site. This additional snow constituted c. 60% of total glacier accumulation. Glacier development was favoured also by the north-east orientation of the site which provided shading from direct solar radiation during the ablation season. In addition to the moraine ridge, a range of smaller depositional landforms exists along the talus foot. These features are younger than the end moraine, but demonstrate that coarse rock debris continued to be produced and transported downslope both during, and after, glacier decay. The escarpment glacier is probably Loch Lomond Stadial in age.

Loch lomond stadial glacier at Fan Hir, Mynydd Du (Brecon Beacons), South Wales: Critical evidence and palaeoclimatic implications

AbstractThe c. 1·2 km long, up to c. 25 m high ridge east of the almost north‐south aligned Fan Hir scarp, Mynydd Du, South Wales has been regarded as a remarkable protalus rampart formed in the Loch Lomond Stadial (c. 11–10000 years BP). New data are presented which indicate that it is a moraine. The main points supporting this glacigenic origin are: its curved plan form at the lower, southern end; its scale and the ample depth for snow to glacier ice transformation; the presence of subsidiary ridges interpreted as recessional moraines; the exceptional rate of rockwall retreat required if it were a protalus rampart; and, most importantly, the presence in the ridge of matrix‐supported abraded clasts, up to 20% of which are striated. Useful criteria for differentiating moraines and protalus ramparts are thereby proposed and a sound basis is provided for palaeoclimatic reconstruction. Palaeoclimatic inferences imply that the glacier owed its existence to the combined effect of a mean July temperature of c. 8·5°C and topographically enhanced accumulation, nearly half of which was from wind‐blown snow.

Reconstructing past glacier dynamics and erosion from glacial geomorphic evidence: Snowdon, North Wales

AbstractBedrock surfaces exposed around Llyn Llydaw, North Wales demonstrate contrasting styles of erosion beneath a Late Devensian ice sheet and a Loch Lomond Stadial (LLS) valley glacier. Ice sheet erosion involved lee‐side fracturing, surface fracture wear and abrasive wear, while LLS erosion was primarily by abrasive wear. Preservation of ice sheet erosional features indicates limited rates of erosion during the LLS. Analysis of the geometry and distribution of erosional markings suggests that the low erosional capacity of the LLS glacier was due to a low basal sliding velocity. This prevented the formation of lee‐side cavities, reduced the debris flux over the bed and minimised particle‐bed contact loads. Reconstructions of the mass balance and geometry of the LLS glacier indicate that most of its balance velocity could be achieved by internal deformation alone. A combination of low subglacial water pressures and an unusually rough substrate explain the low sliding velocities. High bed roughness is due to the absence of leeside cavities and a change in flow orientation between ice sheet and LLS times, which meant that the LLS glacier was in contact with roughness elements which were generated in cavities beneath the ice sheet.

The characteristics and significance of some lateglacial protalus ramparts in upland Britain

AbstractArcuate and linear protalus ramparts of inferred Loch Lomond Stadial age are widespread in upland Britain outside the limits of contemporaneous glaciers. Survey and analysis of the morphology of nine ramparts suggests that these may have formed at the foot of slowly thickening snowbeds, with rockfall debris accumulating against their distal slopes, and that snowbed melting at the end of the stadial was uninterrupted by prolonged periods of stability or renewed growth. Rampart sediments consist of poorly‐sorted assemblages of clasts with a variable infill of fines. Clast size and shape are strongly influenced by lithology, but rampart clasts are characteristically more angular and ‘slabbier’ than those of similar lithology in ice‐marginal moraines. Ramparts may also be distinguished from moraines by their location, morphology, lack of a true matrix of fines and absence of erratics, and from talus‐foot rock glaciers in terms of their width and the absence of flow structures. The mapped distribution of rampart altitudes across the Scottish Highlands displays a marked regional trend, with the lowest features in the west and south and highest in the Cairngorms. This pattern mirrors that of reconstructed firn line altitudes of Loch Lomond Stadial glaciers, and is inferred to reflect a pronounced eastwards and northwards decline in snowfall during the stadial.