Alpine icefield landsystem of upland Britain

This article was written by Dr Hannah Bickerdike.

The Loch Lomond Stadial in Britain

Between 12,900 and 11,700 years ago, gradual warming of Britain’s climate was interrupted by a sudden period of renewed cooling. During this period, known as the Loch Lomond or Younger Dryas Stadial, glaciers regrew in many areas of upland Britain.

Evidence of these glaciers is preserved in a range of different glacial landsystems in Britain. Even though these glaciers have long since disappeared, by studying the Younger Dryas glacial landsystems they left behind, we can understand what processes operated in these glacial environments.

Glacial geomorphology in Scotland dating from the Younger Dryas. Credit: Bickerdike et al., 2018

The alpine icefield landsystem

The most widespread landsystem of the Loch Lomond Stadial is the alpine icefield, evidence of which is found throughout the mountainous areas of the Western Grampian Highlands of Scotland and on several of the Western Isles, including Skye and Mull.

This landsystem is a type of glaciated valley landsystem, usually consisting of a series of steep-sided, glacial valleys, separated by arêtes and spurs. The size and shape of these glaciers was strongly controlled by the topography, with ice confined to within the valleys.

In some places, ice from two or more separate valleys would join together over lower sections of the mountain ridges, called cols. This created networks of connected valley glaciers called icefields.

Conceptual diagram of the alpine icefield landsystem. Glacier size, shape and flow was largely controlled by topography. Adapted from Bickerdike et al., 2018 – https://onlinelibrary.wiley.com/doi/full/10.1111/bor.12259  

Landforms of the alpine icefield landsystem

The diagram below shows the types of landform usually found in the Loch Lomond Stadial alpine icefield landsystem in Britain. The numbered features are discussed below.

Glacial geomorphology of the alpine icefield landsystem. Adapted from Bickerdike et al., 2018 -https://onlinelibrary.wiley.com/doi/full/10.1111/bor.12259

#1. Recessional moraines

The most widespread feature of this landsystem is sequences of recessional moraines, which are arranged in concentric ridges on the valley floors and lower slopes. Moraines are piles of debris, usually mud, sand, and boulders, all deposited in piles at the ice terminus. They are typically unsorted and chaotic.

These moraines formed during short phases of glacier advance and retreat that interrupted the general pattern of glacier retreat and are typical of active temperate glaciers.

Recessional hummocky moraines in Strath Beag, Isle of Skye. Source Bickerdike et al., 2018 – https://onlinelibrary.wiley.com/doi/full/10.1002/jqs.3010

#2. Moraine mounds

Sometimes there are small patches of more chaotically arranged moraine mounds (2) within these sequences. Areas of extensive moraines indicate that the Loch Lomond Stadial glaciers transported large volumes of debris.

Some of this debris likely fell onto the glacier surfaces from the surrounding valley slopes but it is also thought that the glaciers reworked large volumes of debris that was already present in the landscape.

#3. Eskers

In some places, eskers (3) are present on the valley floors, but these are less common. Eskers are ridges of sand and gravel, deposited by glacial meltwater flowing through tunnels within and underneath glaciers. After the glacier disappears, these sediments are left behind as a ridge in the landscape.

A recently formed Esker – sinuous tidge aligned more or less along ice flow. Credit: Frances Butcher

#4. Medial moraines

Similarly, medial moraines (4) may mark locations at the confluence of two valley glaciers, but evidence of these within this landsystem is rare. Medial moraines form where two glaciers met.

#5. Terminal moraines

In some valleys, particularly those with cirques at their heads, recessional moraines are only found in the area around the former glacier terminus (5).

#6. Flutes

In these valleys with cirques at their head, the upper valley might be covered within a thin blanket of till or show evidence of flutes (6). Flutes are streamlined ridges of sediment, sometimes with a boulder or obstacle at their head, that formed subglacially underneath temperate ice.

#7, #8. Erosional landforms

At the heads of these valleys, erosional glacial landforms can often be found. These can include roches moutonées (7), formed by abrasion and quarrying of the bedrock under the sliding glacier, and ice-smoothed bedrock (8).

Roche moutonnée from Scotland, with a gently sloping stoss face and a blunt lee face. Photo: David Baird

#9. Trimlines

In many areas, the height of the former glacier surface is marked by trimlines (9). These features show the height of the former glacier surface on the valley slopes.

Trimlines can be identified by the contrast between glacial landforms below the trimline (in the area covered by the former glacier), and evidence of frost-shattering and periglacial processes above the trimline (in areas that remained above the glacier surface).

Cast study: Isle of Mull Alpine Icefield

The Isle of Mull on the West coast of Scotland shows glacial geomorphology typical of the alpine icefield landsystem. The numbers on the map match with the features described above.

The Isle of Mull had an independent ice domes that deflected mainland ice around it during the Last Glacial Maximum. During the Younger Dryas, it was glaciated with an independent mountain icefield.

Ice drained from the broad uplands of Sgurr Dearg and the Beinn Talaidh-Corra-bheinn ridge to form the Ba and Forsa outlet glaciers to the northwest and north, respectively.

The lower slopes of these valleys are covered with nested lateral moraines, chains of recessional moraines and thick drift of glacial sediments. The terminus of the glaciers is obscured by glaciofluvial outwash sands and gravels.

Example of the Loch Lomond Stadial alpine icefield landsystem on the Isle of Mull, Scotland (originally mapped by Ballantyne 2002). Underlying hill‐shaded images were derived from NEXTMap DSM from Intermap Technologies, Inc. provided by the NERC Earth Observation Data Centre. Adapted from Bickerdike et al., 2018 – https://onlinelibrary.wiley.com/doi/full/10.1111/bor.12259

There were six cirque glaciers around the margins of the icefield. They were not connected to the main icefield.

In summary, the Loch Lomond Stadial alpine icefield landsystem is found in upland areas of Britain with interconnected steep-sided glacial valleys.

The landsystem contains: sequences of recessional moraines on the valley floors and lower slopes (typical of active temperate glaciers); flutings or glacial erosional landforms in the upper valleys; and trimlines marking the former glacier surface.

Activity

You can explore the glacial landforms of Mull using the Younger Dryas Glacial Map.

Use the map to zoom to the Isle of Mull. Zoom in and out and explore the landforms. Turn the basemap to satellite imagery and investigate the geomorphological evidence for yourself. Can you see the features in the satellite imagery?

The Younger Dryas Glacial Map

Further reading

About the Author

Hannah Bickerdike completed her BSc in Geography at the University of St Andrews. She subsequently undertook a PhD at Durham University, studying the geomorphology of the Loch Lomond/Younger Dryas Stadial glaciers of Britain. A key element of this work was compiling geomorphological evidence of these glaciers, mapped in previous research, into a GIS database of over 95,000 features, a version of which can be found on this site.

References

  1. Bickerdike, H. L., Ó Cofaigh, C., Evans, D. J. A. & Stokes, C. R. Boreas 47, 202–224 (2018).
  2. Bickerdike, H. L., Evans, D. J. A., Stokes, C. R. & Ó Cofaigh, C. J. Quat. Sci. 33, 1–54 (2018).
  3. Bickerdike, H. L., Evans, D. J. A., Ó Cofaigh, C. & Stokes, C. R. J. Maps 12, 1178–1186 (2016).

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