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 Scotland^2,3, 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.

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.

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 mass^5,9.

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.

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).

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.

Hummocky drift

Inside terminal moraines, it is common to observe hummocky drift mounds, which display no obvious alignment to a former ice margin^5,6 and may reflect the wastage of ice and/or the chaotic dumping of debris during deglaciation.

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.

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.

Protalus ramparts

Protalus ramparts have the appearance of moraine ridges but were not formed by glacier ice^13,14. 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.

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.

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.