Cirque glaciation landsystem of upland Britain

The Loch Lomond Stadial in Britain

Between around 13 and 11 thousand years ago, the climate in Britain, as well as across much of Northern Europe, cooled abruptly1. This short-lived cold period temporarily reversed the general pattern of warming that drove the retreat of ice sheets after the Last Glacial Maximum, causing glaciers to readvance in many mountain regions.

In Britain, this cold snap is known as the Loch Lomond Stadial. In the Loch Lomond Stadial, an ice cap grew over the western Highlands of Scotland2,3, along with other smaller icefields, valley glaciers, and cirque glaciers that formed in the mountains and uplands of Scotland, England (e.g. Lake District4) and Wales (e.g. Snowdonia5).

Loch Lomond Stadial cirque glaciers

In the Loch Lomond Stadial, cirque glaciers formed in areas that were close to the threshold for glaciation6, such as around the margins of larger icefields, or in areas where the climate was not suited (e.g. warmer melt season temperatures7) to forming larger glaciers, typically further away from the main centre of glaciation in the Scottish Highlands.

Cross-section through an idealised cirque glacier occupying a mountainside hollow.

Cirque glaciers occupied bedrock hollows (cirques) in mountain sides or the lee (downwind) side slopes of escarpments. Cirques with a north or northeasterly aspect were particularly favourable sites for glaciation5 as they protected the ice from direct solar radiation for much of the day, resulting in less ice-melt across the year8.

Map of Loch Lomond Stadial cirque glaciers in Snowdonia, North Wales, showing a strong preference for northeasterly facing ice masses. Image from Bendle & Glasser (2012; ref. 5)

In addition, southwesterly prevailing winds blew snow from mountain summits and plateaus into the cirques below that, along with avalanches from steep cirque sides, added to glacier mass5,9.

In summary, topography played an important role in Loch Lomond Stadial cirque glaciation in Britain and, in turn, the landsystem these cirque glaciers created6.

Landforms created by Loch Lomond Stadial cirque glaciers

The cirque glacier landsystem of upland Britain6 contains landforms created directly by glacier ice, and landforms related to periglacial and paraglacial activity outside the limits of glacier cover.

Inside the limits of glaciation

Moraines

The maximum extent of Loch Lomond Stadial cirque glaciers is typically marked by a terminal moraine6. This may occur as a single, arcuate terminal ridge, or as a small belt of moraines around the maximum ice extent. Sometimes, although not always, recessional moraines extend some way back into the cirque floor, recording active glacier retreat5.

Arcuate, boulder-covered moraine ridges marking the limit of a former cirque glacier in Cwm Cau, mid-Wales. Note the hummocky drift enclosed inside the outer terminal moraine. Note also, talus slopes formed outside the limit of glaciation. Photo: J. Bendle.

Sometimes, terminal moraines are large in comparison to the cirque glacier that formed them. Such large moraines form in two main scenarios. First, where a glacier snout remained stable at a given location for a prolonged period of time6, allowing a large amount of debris to build up around its margin. Second, where glacier advance entrained a large amount of debris from the cirque floor and sides (possibly left behind by earlier glacial and paraglacial activity).

Lateral moraine of a Loch Lomond cirque glacier that formerly filled the Snowdon Horseshoe valley, North Wales. Photo: J. Bendle.

Drift limits

The limit of cirque glaciers is not always marked by moraine ridges5. Sometimes, glacial extent is recorded by ‘drift’ – a fairly homogenous blanket of glacial diamict (‘till’). The drift covered floor of some cirques contrasts greatly with the drift-free slopes above, allowing the vertical thickness of ice to be estimated10.

Glacial drift (the light-coloured ground dotted with boulders) marking the limit of cirque glaciers in Snowdonia, North Wales. Image from Google Earth.

Hummocky drift

Inside terminal moraines, it is common to observe hummocky drift mounds, which display no obvious alignment to a former ice margin5,6 and may reflect the wastage of ice and/or the chaotic dumping of debris during deglaciation.

Hummocky drift mounds (foreground) on the floor of Cwm Idwal, Snowdonia (North Wales) and a chain of lateral moraine ridges (background). Photo: J. Bendle.

Erosional landforms

Along with the depositional landforms described above, the floor and sides of cirques were often eroded by Loch Lomond Stadial glaciers, forming of ice-moulded bedrock, roches moutonnées, and striations5, which show that cirque glaciers were (at least at times) warm based.

Striations (thin grooves running from right to left) scored into bedrock on the floor of a cirque in Snowdonia, North Wales. Photo: J. Bendle.

Outside the limits of glaciation

Summit blockfields and frost weathered debris

Blockfields and frost-weathered debris are commonly found on the mountain summits above cirque basins, and talus slopes often blanket cirque sides above the limit of glaciation. These periglacial features, formed by frost-weathering in extremely cold conditions11, are therefore a useful indicator of the vertical thickness of ice12.

Frost-weathered debris on mountain summits above the limit of Loch Lomond Stadial cirque glaciation in Snowdonia, North Wales. Photo: J. Bendle.
Valley side talus slopes marking the upper (vertical) limit of cirque glaciation in Cwm Idwal, Snowdonia. Photo: J. Bendle.

Protalus ramparts

Protalus ramparts have the appearance of moraine ridges but were not formed by glacier ice13,14. Instead, they formed around perennial snowbeds, where debris weathered from the cirque backwall or sides fell on to the snowbed and slid or rolled downslope to accumulate as ridges around the snowbed margin.

Protalus ramparts – while looking like moraines – form where rock debris falls, rolls, and slides across a perennial snowbed to build up a ridge (or rampart) at the snowbed edge.

Rockslope failures

Rockslopes failures often create moraine-like ridges and/or hummocky deposits that may be mistaken for glacier limits, especially when they occur in cirques15. Rockslope failures are, however, paraglacial features (i.e. features formed by unstable conditions following the retreat of glacial ice from an area16), mostly formed because of high seismic activity caused by postglacial rebound following the last ice sheet glaciation of Britain17.

Boulder-capped rockslope failure from the backwall of Cwm Bochlwyd in Snowdonia, North Wales. Photo: J. Bendle.

The cirque glacier landsystem of the Loch Lomond Stadial

In summary, the cirque glaciation landsystem6 created throughout upland Britain during the Loch Lomond Stadial contains: (1) outer limits marked by moraines and drift, with recessional moraines on some cirque floors indicating active retreat in deglaciation; (2) erosional landforms, such as roches moutonnées and striations that provide evidence of warm based ice; and (3) periglacial (e.g. talus slopes) and paraglacial (rock slope failures) landforms created outside glacier limits.

An example of the Loch Lomond Stadial cirque glacier landsystem at Cwm Idwal, Snowdonia (North Wales). Lateral moraines – and a bouldery ridge beyond Llyn (lake) Idwal that approximates the former glacier terminus – enclose hummocky drift and areas of ice-moulded bedrock. Outside the glacier limits, talus slopes blanket the valley side. Photo: J. Bendle.

References

[1] Rasmussen, S.O., Bigler, M., Blockley, S.P., Blunier, T., Buchardt, S.L., Clausen, H.B., Cvijanovic, I., Dahl-Jensen, D., Johnsen, S.J., Fischer, H. & Gkinis, V. (2014) A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: refining and extending the INTIMATE event stratigraphy. Quaternary Science Reviews106, 14–28.

[2] Golledge, N.R. (2007) An ice cap landsystem for palaeoglaciological reconstructions: characterizing the Younger Dryas in western Scotland. Quaternary Science Reviews 26, 213–229.

[3] Golledge, N.R. (2010) Glaciation of Scotland during the Younger Dryas stadial: a review. Journal of Quaternary Science, 25, 550–566.

[4] McDougall, D.A. (2013) Glaciation style and the geomorphological record: evidence for Younger Dryas glaciers in the eastern Lake District, northwest England. Quaternary Science Reviews, 73, 48–58.

[5] Bendle, J.M. & Glasser, N.F. (2012) Palaeoclimatic reconstruction from Lateglacial (Younger Dryas Chronozone) cirque glaciers in Snowdonia, North Wales. Proceedings of the Geologists’ Association, 123, 130–145.

[6] Bickerdike, H.L., Ó Cofaigh, C., Evans, D.J.A. & Stokes, C.R. (2018) Glacial landsystems, retreat dynamics and controls on Loch Lomond Stadial (Younger Dryas) glaciation in Britain. Boreas, 47, 202–224.

[7] Ballantyne, C.K. (2007) Loch Lomond Stadial glaciers in North Harris, Outer Hebrides, North-West Scotland: glacier reconstruction and palaeoclimatic implications. Quaternary Science Reviews, 26, 3134–3149.

[8] Evans, I.S. 1977. World-wide variations in the direction and concentration of cirque and glacier aspects. Geografiska Annaler: Series A, Physical Geography59, 151–175.

[9] Mitchell, W.A. (1996) Significance of snowblow in the generation of Loch Lomond Stadial (Younger Dryas) glaciers in the western Pennines, northern England. Journal of Quaternary Science, 11, 233– 248.

[10] Ballantyne, C.K. (2002) The Loch Lomond Readvance on the Isle of Mull, Scotland: glacier reconstruction and palaeoclimatic implications. Journal of Quaternary Science, 17, 759–771.

[11] Curry, A., Jennings, S., Scaife, R. & Walden, J. (2007) Talus accumulation and sediment reworking at Mynydd Du. In Carr, S.J., Coleman, C.G., Humpage, A.J. & Shakesby, R.A. (eds.): The Quaternary of the Brecon Beacons: Field Guide, 120–127. Quaternary Research Association, London.

[12] Benn, D.I. & Ballantyne, C.K. (2005) Palaeoclimatic reconstruction from Loch Lomond Readvance glaciers in the West Drumochter Hills, Scotland. Journal of Quaternary Science, 20, 577–592.

[13] Shakesby, R.A. & Matthews, J.A. (1993) Loch Lomond Stadial glacier at Fan Hir, Mynydd Du (Brecon Beacons), South Wales: critical evidence and palaeoclimatic implications. Geological Journal, 28, 69– 79.

[14] Carr, S.J. & Coleman, C.G. (2007) An improved technique for the reconstruction of former glacier mass-balance and dynamics. Geomorphology, 92, 76–90.

[15] Carr, S.J., Coleman, C.G., Evans, D.J.A., Porter, E.M. & Rea, B.R. (2007) An alternative interpretation of Craig y Fro based on mass balance and radiation modelling. In Carr, S.J., Coleman, C.G., Humpage, A.J. & Shakesby, R.A. (eds.): The Quaternary of the Brecon Beacons: Field Guide, 120–127. Quaternary Research Association, London.

[16] Ballantyne, C.K. (2002) A general model of paraglacial landscape response. The Holocene, 12, 371–376.

[17] Ballantyne, C.K., Sandeman, G.F., Stone, J.O. & Wilson, P. (2014) Rock-slope failure following Late Pleistocene deglaciation on tectonically stable mountainous terrain. Quaternary Science Reviews, 86, 144–157.

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