Trimlines are erosional features which mark the maximum vertical extent of a past glaciation [1].

Different types of trimlines exist, dependent on the length of time since the last glacial advance [2]. In recently glaciated valleys, they are marked by a distinct change in vegetation. Above the trimline, dense vegetation exists with species characteristic of the region[2,3]. And below the trimline, valley sides are characteristic of bare, ice-scoured rock, or with early stages of vegetational development [2,3]. These are termed ‘Vegetational Trimlines‘, and are commonly associated with glacial activity since the Little Ice Age, within the historical era [2,3].

For older glaciations, occurring prior to the Little Ice Age, the vegetation change may be less distinctive as vegetation succession has occurred [2,4]. Therefore, these trimlines mark the boundary between the smooth, ice-scoured bedrock below the trimline, and the frost-shattered regolith from periglacial weathering above [3]. These can be termed ‘Peri-glacial Trimlines’ [2,3,5].

Image of the Callequeo Glacier and its terminal lake with trimlines visible from past glacial advance.
Valley side trimlines (labelled with white arrows) marking the former thickness of the Callequeo Glacier, Monte San Lorenzo. Photo credit: J. Martin.

What are Trimlines Used For?

Unlike other glacial landforms which show the lateral extent, or behaviour of a past glaciations, trimlines mark the maximum vertical extent of the ice surface [3,4]. This enables the production of 3-D model reconstructions of ice sheets and valley glaciers[4,6].

In currently occupied glacial regions, trimlines can be compared to the modern ice surface elevation to assess the role of ice surface thinning, and responses to climate change [7].


[1] McCarroll D. (2014) Trimline. In: Encyclopedia of Planetary Landforms. Springer, New York, NY.

[2] Benn, D.I., and Evans, D.J.A., 2010. Glaciers and Glaciation. Hodder-Arnold, London

[3] Rootes, C.M., and Clark, C.D. (2020) Glacial trimlines to identify former ice margins and subglacial thermal boundaries: A review and classification scheme for trimline expression. Earth-Science Reviews. 210. 103355.

[4] McCarroll, D. (2016) Trimline Trauma: The wider implications of a paradigm shift in recognising and interpreting glacial limits. Scottish Geographical Journal. 132(2).

[5] Ballantyne, C. K. (1997). Periglacial Trimline in the Scottish Highlands. Quaternary
International, 38, 119–136.

[6] Ballantyne, C. K. (2010). Extent and Deglacial Chronology of the Last British‐Irish Ice Sheet: Implications of Exposure Dating Using Cosmogenic Isotopes. Journal of Quaternary Science, 25(4), 515–534.

[7] Kohler, J., James, T., Murray, T., Nuth, C., Brandt, O., Barrand, N., Aas, H. & Luckman, A.
(2007). Acceleration in Thinning Rate on Western Svalbard Glaciers. Geophysical Research
Letters, 34(18).


What are flutes?

Glacial flutes are elongated, low-relief ridges formed subglacially and orientated in the direction of glacier flow [1,2,3]. Their size can range between several centimetres to a few metres both wide and high, and occur in groups of streamlined ridges known as ‘swarms’ [1].

Flutes are formed subglacially and are found in glacial foregrounds. They are more likely to be found in modern glacier foreground as they can be subjected to erosion because of their till-like composition [1].  

Fluted surface in the Brúarjökull foreground 2004. Source: Ólafur Ingólfsson

Flutes are found in a variety of glaciated regions including Iceland, Sweden, Norway, New Zealand, and Alaska [1,2,3]. Because of their relatively small size, they are often hard to identify whilst at ground level. Therefore high-resolution satellite data or LiDAR methods are used to map them [2].

Flute formation

Flutes can be formed subglacially beneath both polythermal, and warm-based glaciers. There have been several models proposed about the formation of flutes, but the most widely accepted model is the Cavity Infill Model [1,2,4].

This model proposes that a boulder causes an obstruction beneath the actively flowing glacier. The glacier then forces highly saturated sediment into a cavity on the leeside of the boulder obstruction [1,2]. The pressure on the leeside of the boulder drops, allowing the saturated sediment to freeze, and is carried forward by the ice, forming the elongated flute shape [1,3].


[1] Benn, D.I., and Evans, D.J.A., 2010. Glaciers and Glaciation. Hodder-Arnold, London

[2] Ely, J.C., Graham, C., Barr, J.D., Rea, B.R., Spagnolo, M., and Evans, J. (2016) Using UAV acquired photography and structure from motion techniques for studying glacier landforms: application to the glacial flutes at Isfallsglaciären. Earth Surface Processes and Landforms. DOI: 10.1002/esp.4044.

[3] Gordon, J.E., Whalley, W.B., Gellatly, A.F., Vere, D.M. (1992) The formation of glacial flutes: Assessment of models with evidence from Lyngsdalen, North Norway. QSR. 13(7). PP. 709-731.

[4] Boulton, G.S. (1976) The origin of glacially fluted surfaces – observations and theory. Journal of Glaciology. 17. PP. 287-309.

Tidewater Glaciers

What is a Tidewater Glacier?

Tidewater glaciers are glaciers which extend out, and terminate into the sea [1]. They are part of a group of glaciers known as calving glaciers, as their main method of ice loss is through iceberg calving, instead of surface melt [1,2]. Calving icebergs currently accounts for up to 70% of the worlds annual mass transfer from glacial regions to the ocean [1].

Whisky Glacier, a floating tidewater glacier in Whisky Bay, James Ross Island

Tidewater glaciers are found at latitudes of 45 and above, and are present in different glacial regions including Antarctica, Alaska, Greenland, Svalbard, and Patagonia [1,2].

Types of Tidewater Glacier

Mountain glaciers terminating into the ocean are called ‘tidewater glaciers’. ‘Tidewater outlet glaciers’ are glaciers which reach the ocean through fjords, branching off from ice caps, ice sheets or icefields [1].

Tidewater glaciers can either be grounded – where the glacier is in constant contact with the bed. Or they can be floating – when the terminus is floating on the sea water, or flowing into an ice shelf [1,3]. Grounded glaciers tend to be located in temperate regions such as Alaska, or Canada. And floating tidewater glaciers are commonly found in polar regions, namely Greenland, Svalbard, and Antarctica.

Tidewater Glaciers and Iceberg Calving

Calving icebergs are the most efficient method of losing mass from a glacier [4]. It is the dominant cause of mass loss from the Antarctic Ice Sheet [5], therefore, it is important to understand the process behind these calving events [1,4]

Iceberg calving occurs when there are faults in the glacier, known as crevasses. Crevasses can form when there stress and strain thresholds are reached on the glacier. The trigger for the iceberg calving events vary for both grounded and floating tidewater glaciers.

A tidewater glacier with crevasses calving icebergs

Floating tidewater glacier

For a floating tidewater glacier, submarine melting of the underside of the glacier causes a direct loss of ice, as well as undercutting the floating glacier terminus or ice shelf [6]. This causes instability, resulting in complete collapse [1,6].

When the floating section of the tidewater is removed, the ice on the land is no longer supported from the buttressing ice shelf. It is then able to rapidly, and continuously calve icebergs. For example, the Larsen B ice shelf collapse in 2002 on the Antarctic Peninsula [4].

Grounded tidewater glaciers

Grounded tidewater glaciers calve when there is either a rapid thinning of the glacier surface, or a localised change in sea level. This change forces the glacier terminus to be out of equilibrium with the ocean, resulting in the terminus to be lifted and detached from the bed, causing the terminus to become buoyant [6]. During this process, the crevasses are able to isolate large blocks of ice which are then calved into icebergs.

Illustration of a grounded glacier during a calving event.


[1] Vieli, A., 2011. Tidewater glaciers, in: Singh, V.P., Singh, P., Haritashya, U.K. (Eds.), Encyclopedia of Snow, Ice and Glaciers. Springer, pp. 1175–1179.

[2] Benn, D.I., Hulton, N.R.J., Mottram, R.H., 2007. “Calving laws”, “sliding laws” and the stability of tidewater glaciers, in: Sharp, M. (Ed.), Annals of Glaciology, Vol 46, 2007, Annals of Glaciology. Int Glaciological Soc, Univ Ctr Svalbard UNIS, NO-9171 Longyearbyen, Norway. Benn, DI, Univ Ctr Svalbard UNIS, Box 156, NO-9171 Longyearbyen, Norway., pp. 123–130.

[3] van der Veen, C.J., 2002. Calving glaciers. Prog. Phys. Geogr. 26, 96–122.

[4] Benn, D.I., and Evans, D.J.A., 2010. Glaciers and Glaciation. Hodder-Arnold, London

[5] Shepherd, A., Ivins, E., Rignot, E., Smith, B., Van Den Broeke, M., Velicogna, I., Whitehouse, P., Briggs, K., Joughin, I., Krinner, G., IMBIE Team, 2018. Mass balance of the Antarctic Ice Sheet from 1992 to 2017. Nature 558, 219–222.

[6] Benn, D.I., Astrom, J., Zwinger, T., Todd, J., Nick, F.M., Cook, S., Hulton, N.R., and Luckman, A. (2017) Melt under-cutting and buoyancy-driven calving from tidewater glaciers: new insights from discrete element and continuum model simulations. Journal of Glaciology. 63(240).

Younger Dryas glacial moraines (Lake District)

By Dr Richard Waller, Keele University, and Dr Bethan Davies, Royal Holloway University of London

During the Younger Dryas, the Lake District was covered by plateau icefields and cirque glaciers[1]. The image below shows the larger plateau icefields (green) and the smaller cirque glaciers (red) in the Lake District and Snowdonia.

Ice masses in Wales and the Lake District. From Bickerdike et al., 2018

You can explore all of the locations in this page using the Younger Dryas Glacial Map. This version of the map is focused on the Lake District.

Younger Dryas Glacial Map – Lake District

Greenup Gill

These ice masses left behind numerous moraines as they retreated. The image below (credit Dr Richard Waller) shows some 360º imagery of a fabulous set of hummocky moraines in Greenup Gill, near Borrowdale, in the Central Lake District[3]. These glacial landforms show the retreat of a small plateau icefield outlet glacier during the Loch Lomond Stadial[1,3].

The moraines show the retreat of the glacier all the way up onto the plateau, showing the active retreat of this plateau icefield. The continuous moraine sequences shows that the outlet glacier retreated towards its plateau source area without becoming disconnected from the plateau icefield accumulation area[2].

You can see the mapped moraines of this icefield for yourself in the Younger Dryas Glacial Map.

Greenup Gill moraines (dark grey) and the plateau icefield (light gray) from the plateau icefield, Younger Dryas Glacial Map.


This 360º image shows a lateral moraine (a moraine formed at the sides of the glacier) in Bannerdale (credit Dr Richard Waller). Bannerdale held a small cirque glacier during the Younger Dryas, with lateral moraines demarcating the glacier limits.

In the image below, the lateral moraine is visible as a linear mound of sediment against the valley side walls, in the immediate foreground. Some glacially transported boulders are visible on the ridge of the moraine.

Bannerdale, just to the east of Blencathra, held small cirque glaciers during the Younger Dryas. The orange in this map is mapped as ‘hummocky moraine’ by Sissons (1980).


Haweswater also held a plateau icefield during the Younger Dryas [1,3].

Younger Dryas Glacier Map, showing plateau icefield above Haweswater.

This is a set of moraines above Haweswater (credit Dr Richard Waller). The moraines track the recession of the plateau icefield outlet glaciers onto the upland areas [5].

You can explore these moraines yourself in Google Earth or in the Younger Dryas Glacial Map. The moraines are visible in the satellite imagery as rounded hummocks with scattered boulders.

The Younger Dryas Glacial Map shows the locations of these moraines, just at the head of Haweswater Reservoir.

Younger Dryas moraines at the head of Haweswater Reservoir


Gillercomb, the valley just to the west of Seathwaite (Cumbria), preserves a number of glacier moraines deposited during the Younger Dryas.

Gillercomb, west of Seathwiate

The location can be explored in Google Maps. The moraines are visible as the smoothed, elongated mounds in the valley floor.

These moraines were formed during the recession of the plateau icefield that covered this part of Cumbria during the Younger Dryas [6].

Gillercomb, west of Seathwaite in Cumbria. Younger Dryas Glacial Map.

Grains Gill

A number of small elongate moraines exist in the bottom of the Derwent river valley, just south of Seathwaite. These moraines have a number of glacially transported boulders on their summits.

Screenshot from the Younger Dryas Glacial Map of Grains Gill moraines.

Here is the location of the mapped geomorphology:

Further reading


1. Bickerdike, H. L., Ó Cofaigh, C., Evans, D. J. A. & Stokes, C. R. Boreas 47, 202–224 (2018).

2. Boston, C. M. & Lukas, S. J. Quat. Sci. 34, 433–451 (2019).

3. McDougall, D. A. J. Quat. Sci. Publ. Quat. Res. Assoc. 16, 531–543 (2001).

4. Sissons, J. B. Earth Environ. Sci. Trans. R. Soc. Edinburgh 71, 13–27 (1980).

5. McDougall, D. Quat. Sci. Rev. 73, 48–58 (2013).

6. McDougall, D. A. (1998). Loch Lomond stadial plateau icefields in the Lake District, northwest England. PhD thesis, University of Glasgow.

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 –  

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 -

#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 –

#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 –

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.


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.


  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).

From Snow to Firn to Glacier ice


How do we build a glacier? We start with a snowflake. Snow, over time, is compressed into firn, and then into glacier ice.

Snow falls in cold regions, such as mountain tops or in polar regions. In glaciology, snow refers to material that has not changed since it fell1.

Snow is very light and fluffy, and has a very low density. If the snow is wetter, it will have an increased density. Snowflakes have a hexagonal structure, and fallen snow has a significant amount of air in it.

Snow flakes by Wilson Bentley. Bentley was a bachelor farmer whose hobby was photographing snow flakes. ; Image ID: wea02087, Historic NWS Collection ; Location: Jericho, Vermont ; Photo Date: 1902 Winter. From Wikimedia Commons


Firn is usually defined as snow that is at least one year old and has therefore survived one melt season, without being transformed to glacier ice.

Firn is transformed gradually to glacier ice as density increases with depth, as older snow is buried by newer snow falling on top of it. Year after year, successive accumulation layers are built up. In the accumulation zone of a glacier, density therefore increases with depth; the rate depends on the local climate and rate of accumulation1. Firn transforms to glacier ice at a density of 830 kg m-3.

New snow (immediately after falling, calm conditions50-70
Damp new snow100-200
Settled snow200-300
Wind-packed snow350-400
Very wet snow and firn700-800
Glacier ice830-923
Typical densities (kg m-3). From Cuffey and Paterson, 2010.
A scientist collecting snow and ice samples from the wall of a snow pit. Fresh snow can be seen at the surface and en:glacier ice at the bottom of the pit wall. The snow layers are composed of progressively denser en:firn. Taku Glacier, Juneau Icefield, en:Tongass National Forest, en:Alaska. From Wikimedia Commons

Firn transforms to glacier ice in 3-5 years in the temperate Upper Seward Glacier in the St Elias Mountains near the Alaska-Yukon border. Firn becomes ice at a depth of about 13 m1. At sites like this with rapid snow accumulation, the depth of a firn layer, and the load on it, increases rapidly with depth.

However, in cold, dry East Antarctica, firn becomes ice at a depth of 64 m at Byrd and 95 m at Vostok. 280 years are needed at Byrd, and 2500 at Vostok. Low temperatures slow the transformation. Temperatures at Vostok, the coldest region of Earth, are 30°C lower than Byrd, which explains the slower increase in density. In addition, slow accumulation gives slow burial, and a small load each year; the increase in density takes much longer.

Typically, the transformation of firn to ice takes 100-300 years, and a depth of 50 – 80 m1.

Glacier ice

Firn becomes glacier ice when the interconnecting air or water-filled passageways between the grains are sealed off (“pore closure”)1. Air is isolated in separate bubbles. This occurs at a density of 830 kg m-3. The air space between particles is reduced, bonds form between them, and the particles grow larger. This is a process known as sintering. Increasing pressure compresses the bubbles, placing the enclosed air under pressure and increasing the density of the ice2.

Fresh snowflakes, which have a complex shape, have a large surface area. Over time and under pressure, the surface area is reduced, the surface is smoothed, and the total surface area is reduced. Fresh, complex snowflakes are transformed into rounded particles.

Formation of glacier ice. Luis Maria Benitez, Wikimedia Commons

The transformation of firn to ice is much faster where there is melting and refreezing2.  Meltwater can percolate downwards, infilling porespaces, and the displaced air escapes upwards. If the snow is under 0°C, the water will freeze, producing areas of compact ice.  This will produce high density ice much more rapidly than in colder regions without melting.

The density of pure glacier ice is usually taken as 917 kg m-3. This strictly is only true at 0°C and in the upper layers of ice sheets and mountain glaciers; the density may be greater at the mid-depth ranges in polar ice sheets, where there are no bubbles and temperatures are -20°C to -40°C1.

Below 4 km of ice, such as at the centre of the East Antarctic Ice Sheet, the pressure would increase the density to 921 kg m-3.


Bubbles are common in glacier ice. Bubbles can contain liquid water or atmospheric gases, making them very useful for ice core research. The air in the bubble largely reflects the atmospheric concentrations when the ice formed1. In polar environments, bubbles in the ice occupy about 10% of the volume when firn turns to ice.

Glacier ice with many bubbles exposed on the ice shelf. It is melting and thinning rapidly.
Close up of white bubble-rich ice. Note the sharp junction between the coarse-clear ice and bubble-rich ice.

With greater depth in polar ice sheets, bubbles shrink as the overlying ice increases. The gas pressure within the bubbles therefore increases, and at certain depths, the gas attains a dissociation pressure. The bubbles begin to disappear as the gas molecules form clathrate hydrates1.  This process takes thousands of years.


Glacier ice contains various impurities in tiny amounts. By most scales, glacier ice is a very pure solid-earth material, because the processes leading to snowfall on a glacier – evaporation, condensation, precipitation – act as a natural distillation system1.

However, glaciers can contain impurities. The dirtiest glaciers are mountain glaciers, where debris can fall directly onto the ice surface. On ice sheets and glaciers, dust and other debris may blow onto the ice surface.

Iceberg laden with debris from a glacier, Antarctic Peninsula

Debris on the ice surface can affect the absorption of energy at the ice surface, and lead to increased or decreased melting.

Supraglacial debris on Unnamed Glacier, James Ross Island, Antarctic Peninsula

Layers in the ice

Glaciers are composed of sedimentary layers in their accumulation zones, formed of annual layers of snowfall. These layers are initially parallel to the glacier surface. This is the primary stratification in structural glaciology.

In temperate and subpolar settings, the annual sedimentary layers consist of alternating thick layers of bubble-rich ice, which originated as winter snow, and thin layers of clear ice, which are the refrozen meltwater from the summer melt season.

Glacier ice exposed in an ice-cored moraine. Note the foliation with coarse clear ice and white bubble-rich ice.
Primary stratification on a glacier on James Ross Island, Antarctic Peninsula.

Debris horizons may form, when summer melting concentrates debris (such as rockfall and wind-blown dust) on the ice surface.

In cold polar regions, annual layering forms instead by seasonal variation of snow metamorphism and wind deposition1.

This 19 cm long of GISP2 ice core from 1855 m depth shows annual layers in the ice. This section contains 11 annual layers with summer layers (arrowed) sandwiched between darker winter layers. From the US National Oceanic and Atmospheric Administration, Wikimedia Commons.

Blue glacier ice

Glacier ice is blue because the longer visible wavelengths are absorbed. The more energetic, blue, wavelengths are scattered back2.  The effect is greatest with deep, basal ice, which is bubble free and has large crystals. The blue colour tends therefore to be most intense in the calls of calved icebergs or fresh fractures.

Rough, weathered ice and fresh snow will appear white because preferential absorption does not occur.

This iceberg is formed from basal glacier ice. It is blue an has basal dirt. Differential melting forms holes all over its surface.

Further reading


1.           Cuffey, K. M. & Paterson, W. S. B. The Physics of Glaciers, 4th edition. (Academic Press, 2010).

2.           Benn, D. I. & Evans, D. J. A. Glaciers & Glaciation. (Hodder Education, 2010).

Glacial ArcGIS Stories

There are many ArcGIS Story Maps around. Some are better than others; some take too long to load or are not well thought through. But some are excellent.

Who owns Antarctica? Antarctic geopolitics

This excellent and well presented, professionally built StoryMap illustrates who owns Antarctica and introduces the Antarctic Treaty.

Territorial claims of Antarctica.

Glaciation: past, present and future

This great ESRI Storymap introduces glaciers in the present day and at the Last Glacial Maximum. It uses shapefiles from resources like GLIMS or the Randolph Glacier Inventory, and the Global LGM shapefiles from Ehlers and Gibbard. It’s well made and up to date. It also uses the BRITICE map to introduce glacial landforms across the British Isles and the LGM and Younger Dryas in Britain.

Glacier lake hazards in Alaska

This ArcGIS Story by the Alaska Climate Science Centre is better than most. It’s all about glacier hydrology and glacier lakes in Alaska. The videos and pictures are well made, and interspersed with explanatory figures.

Alaska Climate Science Centre ArcGIS Story Map

Disappearing Glaciers

A nice, well illustrated, introduction to glacier recession and mapping glacier change over recent decades.

Disappearing Glaciers Story Map

The recession of Glacier National Park Glaciers

This is a great introduction to using imagery to track and map glacier recession from 1966 to the present day.

Recession of Glacier National Park Glaciers

Glacial Landforms Story Map

This ESRI Story Map introduces a host of glacial landforms. It was a little slow to run for me, though.

Glacial Landforms Story Map

An Introduction to Sea Ice

A lovely storymap that introduces and illustrates sea ice, with illustrations of how it changes with the seasons at both poles.

ESRI StoryMap: An Introduction to Sea Ice

Mapping Mount Everest

This storymap, by Alex Tait from the National Geographic Society, tells us all about Mount Everest and how we map it, with some beautiful graphics.

Mapping Mount Everest

Glacial Landforms of Snowdonia

A straightforward storymap that highlights the glacial landforms in Snowdonia.

Snowdonia Story Map

Connecticut’s landscape is the story of glaciers

Learn about the glacial landforms of Connecticut.

Connecticut’s glacial landscape

The River Tees from Source to Mouth

More fluvial than glacial, but this is a very nice storymap that covers the River Tees. It is suitable for pre-16 as well as post-16 education.

Further reading


This page was contributed by Dr Frances Butcher from Sheffield University.

What is an esker?

Eskers are ridges made of sands and gravels, deposited by glacial meltwater flowing through tunnels within and underneath glaciers, or through meltwater channels on top of glaciers. Over time, the channel or tunnel gets filled up with sediments. As the ice retreats, the sediments are left behind as a ridge in the landscape.

Esker in the foreland of Hørbyebreen, Svalbard. Panel A looks down the esker from its headward end, which is marked with an arrow in panel B. Panel B shows another view of the esker from its side, and the glacier that formed it in the distance. Image credit: Jakub Ondruch. Based on Storrar et al.1
A cross-sectional exposure through a branch of the Kinnity esker adjacent to Knockbarron Wood, Co. Offaly, Ireland (53.1114°N, 7.7435°W). Image credit: Frances Butcher

Eskers are important, because they can tell us about how ice sheets and glaciers behaved. They can tell us about meltwater, and help us reconstruct the former ice surface, and the orientation of the glacier’s snout.

What do eskers look like?

Eskers are usually metres to tens of metres high, and tens to hundreds of metres wide e.g., 2,3. In cross-section, their shape can be sharp-crested (triangular), round-crested (semi-circular), flat-topped (trapezoid), or multi-crested (having two or more crests).

Changes in cross-sectional shape along an esker in Finland. The colours show elevation
as measured from aerial Light Detection and Ranging (LiDAR) at 2m horizontal resolution. The underlying shaded-relief map shows the shape of the land surface as generated from the elevation values, and is artificially illuminated from the top right of the image. Data credit: National Land Survey of Finland

Eskers can range in length from hundreds of metres to hundreds of kilometres. The individual esker ridges that formed beneath the huge, continental-scale ice sheet that covered North America, for example, can extend up to ~100 km in length. Groups of aligned ridges can form fragmented esker chains up to ~300 km long4. Similarly long eskers in Scandinavia were formed by the Eurasian Ice Sheet.

Why are eskers important?

Eskers that formed in subglacial tunnels are valuable tools for understanding the nature and evolution glaciers and ice sheets. They record the paths of basal meltwater drainage near to the ice margin.

The weight of the overlying ice means that the subglacial meltwater is under high pressure. It can therefore flow uphill! This means that, on a local scale, eskers commonly go uphill and climb up local topography.

An esker climbing over hills (including drumlins recording an earlier right-to-left ice flow direction) in Finland. The colours show elevation as measured from aerial Light Detection and Ranging (LiDAR) at 2m horizontal resolution. The underlying shaded-relief map shows the shape of the land surface as generated from the elevation values, and is artificially illuminated from the top right of the image. Data credit: National Land Survey of Finland.

The path taken by the pressurised meltwater in subglacial channels is controlled mostly by the slope of the ice surface, rather than the slope of the bed. Eskers therefore tend to be oriented parallel to ice flow, and transverse to the ice terminus. As a result, the path of an esker section can be used to reconstruct the slope of the ice surface, and the orientation of the ice terminus at the time of its formation.

An esker in Finland, which terminates in a major ice-marginal moraine deposited by the Eurasian Ice Sheet. The ice terminus (white dashed line) was parallel to the moraine, and perpendicular to the esker. Ice flow was from bottom right to top left. The ice sheet terminated in a large ice-dammed lake when the moraine formed5. Where the esker entered this lake, it deposited a sediment fan, which forms part of the moraine. The colours show elevation as measured from aerial Light Detection and Ranging (LiDAR) at 2m horizontal resolution. The underlying shaded-relief map shows the shape of the land surface as generated from the elevation values, and is artificially illuminated from the top right of the image. Data credit: National Land Survey of Finland.

Eskers produced by the last North American and Eurasian Ice Sheets probably record the final retreat of those ice sheets as climate warming increased the rate of meltwater production towards the end of the Pleistocene. Therefore, by studying eskers, we can better understand how glaciers and ice sheets respond to climate warming.

These palaeoglaciological insights are essential for predicting the responses of the contemporary Antarctic and Greenland Ice Sheets to human-induced climate change, and their potential contributions to sea level rise.

Where do eskers form?

Eskers are abundant across the land that was once covered by the former North American (Laurentide) Ice Sheet6, the Eurasian Ice Sheet6, and the British-Irish7 Ice Sheet. You can explore British Ice Sheet eskers using Britice map.

A map of large eskers in Canada, which were deposited by the former Laurentide Ice Sheet. Redrawn by Butcher 2019(8) from Storrar et al. 2013(6).

Subglacial eskers that formed in subglacial meltwater channels (termed R-channels, which are incised upwards into the basal ice) are the most common among those preserved on palaeo-ice-sheet beds. Good examples of more recently formed eskers are seen, for example, at Breiðamerkurjökull in Iceland2, and Høybyebreen in Svalbard8.

In the embedded Google Map below, look for the raised ridges of the eskers that formed in front of Breiðamerkurjökull. The zig-zagging eskers are largely in the direction of flow, whereas the moraines are parallel to the ice margin.

Eskers on paleo-ice-sheet beds are more abundant in areas of crystalline bedrock with thin coverings of surficial sediment than in areas of thick deformable sediment e.g., 9,4. This is because meltwater flowing at the bed is more likely to incise upwards into the ice to form an R-channel where the bed is hard; where the bed is deformable, meltwater is more likely to incise downwards10.

How long does it take to form an esker?

The timescales over which eskers form is a key topic of ongoing debate. Long eskers extending hundreds of kilometres over paleo-ice-sheet beds are not thought to have formed ‘synchronously’ i.e. at a single moment in time in continuous conduits extending deep into ice sheet interiors. Rather, their formation is thought to have been ‘time-transgressive’, with eskers ‘growing’ at their headward ends as their parent glaciers and associated meltwater conduits retreat across the landscapee.g., 11,12.

Under this mechanism, meltwater conduits need not extend more than a few tens of kilometres into the ice interior, beyond which the weight of the overlying ice would make it hard to form stable drainage conduits. Long esker systems may therefore take hundreds to thousands of years to form12

Shorter eskers (hundreds of metres to tens of kilometres in length) could form synchronously, possibly over periods of days-to-weeks, during high-magnitude drainage events such as glacial outburst floods13,14.

About the Author

Dr Frances Butcher is a planetary scientist researching glaciers on Earth and Mars. She completed her PhD entitled ‘Wet-Based Glaciation on Mars’ at the Open University (UK) in 2019. She is currently a member of the European Research Council (ERC) funded PALGLAC project at The University of Sheffield (UK), using glacial landforms on Earth to reconstruct the dynamics of the former Scandinavian Ice Sheet. Frances has been involved in preparations for the ESA-Roscosmos ExoMars (‘Rosalind Franklin’) Rover mission, which launches to Mars in 2022.

You can follow Frances on Twitter @fegbutcher

You can also follow the PALGLAC project @palglac

Dr Frances Butcher


1.         Storrar, R. D. et al. Equifinality and preservation potential of complex eskers. Boreas 49, 211–231 (2020).

2.         Storrar, R. D., Evans, D. J. A., Stokes, C. R. & Ewertowski, M. Controls on the location, morphology and evolution of complex esker systems at decadal timescales, Breiðamerkurjökull, southeast Iceland. Earth Surf. Process. Landf. 40, 1421–1438 (2015).

3.         Perkins, A. J., Brennand, T. A. & Burke, M. J. Towards a morphogenetic classification of eskers: Implications for modelling ice sheet hydrology. Quat. Sci. Rev. 134, 19–38 (2016).

4.         Storrar, R. D., Stokes, C. R. & Evans, D. J. A. Morphometry and pattern of a large sample (>20,000) of Canadian eskers and implications for subglacial drainage beneath ice sheets. Quat. Sci. Rev. 105, 1–25 (2014).

5.         Stroeven, A. P. et al. Deglaciation of Fennoscandia. Quat. Sci. Rev. 147, 91–121 (2016).

6.         Storrar, R. D., Stokes, C. R. & Evans, D. J. A. A map of large Canadian eskers from Landsat satellite imagery. J. Maps 9, 456–473 (2013).

7.         Clark, C. D. et al. BRITICE Glacial Map, version 2: a map and GIS database of glacial landforms of the last British–Irish Ice Sheet. Boreas 47, 11-e8 (2017).

8.         Butcher, F. E. G. Wet-Based Glaciation on Mars. (The Open University, 2019).

9.         Rattas, M. Spatial Distribution and Morphological Aspects of Eskers and Bedrock Valleys in North Estonia: Implications for the Reconstruction of a Subglacial Drainage System Under the Late Weichselian Baltic Ice Stream. Geol. Soc. Finl. Spec. Pap. 46, 63–68 (2006).

10.       Clark, P. U. & Walder, J. S. Subglacial Drainage, Eskers, and Deforming Beds Beneath the Laurentide and Eurasian Ice Sheets. Geol. Soc. Am. Bull. 106, 304–314 (1994).

11.       Hooke, R. L. & Fastook, J. Thermal conditions at the bed of the Laurentide ice sheet in Maine during deglaciation: implications for esker formation. J. Glaciol. 53, 646–658 (2007).

12.       Storrar, R. D., Stokes, C. R. & Evans, D. J. A. Increased channelization of subglacial drainage during deglaciation of the Laurentide Ice Sheet. Geology 42, 239–242 (2014).

13.       Burke, M. J., Woodward, J., Russell, A. J., Fleisher, P. J. & Bailey, P. K. The sedimentary architecture of outburst flood eskers: A comparison of ground-penetrating radar data from Bering Glacier, Alaska and Skeiðarárjökull, Iceland. Geol. Soc. Am. Bull. 122, 1637–1645 (2010).

14.       Burke, M. J., Brennand, T. A. & Perkins, A. J. Transient subglacial hydrology of a thin ice sheet: insights from the Chasm esker, British Columbia, Canada. Quat. Sci. Rev. 58, 30–55 (2012).

The Younger Dryas Glacial Map

Welcome to the Younger Dryas Glacial Map! Here, you can explore the glaciation of the UK during the Younger Dryas glaciation. In the UK, this period is also called the “Loch Lomond Stadial”.

At this time (12,900 to 11,700 years ago), there was a period of abrupt cooling. Glaciers began to grow again in much of upland Britain. There was a large ice field, running the length of the Western Highlands in Scotland. This icefield was surrounded by numerous smaller icefields, ice caps, valley glaciers and cirque or niche glaciers. Glaciers also grew in Snowdonia, the Brecon Beacons, the Lake District, and on the Hebridean Islands.

Glaciation of Britain during the Loch Lomond (Younger Dryas) Stadial. From Bickerdike et al., 2018

Younger Dryas Glacial Map

This glacial readvance left behind a very distinctive geomorphological imprint on the UK. You can explore these data using our Younger Dryas Glacial Map! This is an ArcGIS Online Map that shows the geomorphological evidence for glaciation and the reconstructed glaciers and ice caps.

The Younger Dryas Glacial Map includes all the geomorphological information relating to the Younger Dryas glaciation of the UK. The data were compiled in a series of papers by Bickerdike et al. (2016, 2018a, 2018b). Here, we have hosted these shapefiles on ArcGIS Online and made them publically available. Click the image below to launch the Younger Dryas Glacial Map.

Click the image to launch the Younger Dryas Glacial Map

How to use the map

Launch the Younger Dryas Glacial Map by clicking here.

Read the information on the Spash Screen and then say OK. You will see a map of the UK (as the figure above).

Zoom in and out

You can zoom in on a selected area by pressing Shift and drawing a box with your mouse (SHIFT and DRAG). Click the Home icon on the left hand tool bar to return to the default extent. You can also zoom by scrolling with your mouse, or by using the + and – buttons.

Tools and toolbars

There are a number of icons in the left hand corner. You can hover over them in the webmap for an explanation.

Icons in the Younger Dryas Glacial Map.

On the vertical bar, the + and – buttons allow you to zoom in and out, and the House returns you to the full map extent. The circle shows you where you are. The square box makes the map full-screen; press escape to return. The left and right arrows take you to the past or next extent.

The five horizontal buttons are, in turn, Measure, Basemap, Legend, Layer, and Information (same as the Splash screen). You can turn layers on and off using Layer List, and fiddle with their transparency and other settings.

You can view the Metadata by clicking on the three little dots to the right of each layer in the Layer List.

Change the Basemap

If you wish, you can change the Basemap to a digital terrain model, satellite imagery, or some other kind of basemap. Change it to “Light Grey Canvas” to speed up drawing time.

Try zooming in, and then changing the Basemap to “Satellite Imagery” to see high-resolution satellite imagery of your field of view.

Choose “Terrain with Labels” to see a digital terrain model.

Popup information

Try zooming in over some features on an area of the map (press shift and draw a square with the mouse). Clicking a feature will select it, and bring up a popup. The Popup has the attribute information (including the reference of the original authors who first mapped the feature), and a description and photograph of a typical example.

Popup for Hummocky Moraine, with the satellite imagery basemap

Attribute tables

Zoom to a new area and once the map has loaded, click the little black tab at the base of the screen. This will bring up the Attribute Table for all the layers. By default, they are filtered to the map extent. You can therefore view the attribute data held for each layer in the current map view.

Try zooming over a small area and viewing the attribute data for each type of landform.

Attribute table for Moraine Ridges in the current view.

Select Tool

The Select Tool is in the top right corner. You can use this tool to select different landforms, and then view them in the attribute table. You may wish to turn Basemap to “Light Grey Canvas” if drawing time is slow.

Tick the check box in the Select Tool drop-down menu for the layer that you want to select. Then draw a box with your mouse to select some features.

You can view the selected features in the Attribute Table, by clicking on the three little dots on the right of the Select drop-down menu.

Using the Select tool

Geomorphological Data in the Younger Dryas Map

The glacier outlines were reconstructed in Bickerdike et al (2018a) using a number of key landforms: moraines, meltwater channels, drift limits, and trimlines. From these data, Bickerdike et al. (2018a) reconstructed the ice limits and the extent of ice-dammed lakes. The geomorphological evidence has been organised into a series of shapefiles, each containing a different landform type.

The database contains over 95,000 individual features, which are organised into thematic layers and each attributed to its original citation. The evidence includes moraines, drift and boulder limits, drift benches, periglacial trimlines, meltwater channels, eskers, striations and roches moutonneés, protalus ramparts and ice-dammed lakes.

Younger Dryas Glacial Geomorphology in Scotland (from Bickerdike et al., 2018a).

You can view the entire geomorphological database in this PDF map (Bickerdike et al., 2016).


There are three moraine shapefiles in the Younger Dryas Glacial Map. Moraine ridges are the most abundant; these are the moraine ridges that were deposited at the terminus of glaciers during the Younger Dryas glaciation of Britain.

Some researchers represent the shape and distribution of individual moraine mounds as polygons, whilst others record only ridge crests as line features or just general areas of ‘hummocky moraine’. These different styles of mapping cannot be reconciled within a single layer in ArcMap, and so are split into separate layers accordingly (‘Moraines (detail)’, ‘Moraine_Ridges’ and ‘Moraine_Hummocky_Area’).

Generally, in areas where mapping of individual features overlapped areas of general ‘hummocky moraine’, only the detailed features were digitised to prevent the database becoming too cluttered.

Younger Dryas moraines in Coire Ardair, Scotland

Drift Limits

Drift limits denote the extent of glacier till or sediment that was directly deposited by the glacier. Drift limits can be used alongside moraines to demarcate the maximum extent of the ice limits.

Younger Dryas drift limits. From Bickerdike et al. 2018a


Trimlines form at the ice surface in valleys; the area below the trimline was covered in glacier ice and wasa subjected to glacial erosion. The area above the trimline was exposed above the ice surface, and subjected to frost-shattering and periglacial processes. This leads to a distinct difference in appearance in bedrock above and below the trimline. This can help to reconstruct the ice surface.

Valley side trimlines (labelled with white arrows) marking the former thickness of the Callequeo Glacier, Monte San Lorenzo. Photo credit: J. Martin.

Meltwater channels

Meltwater channels were formed by the passage of water and can form under the glacier (subglacial), in front of the glacier (proglacial) or on the valley side, between the glacier and any valley sides (ice-marginal).

Ice-marginal meltwater channels can extend up onto plateaus, where they form extensive networks. Ice-marginal meltwater channels often form where the ice is thin and cold-based, and frozen to the substrate. Subglacial drainage was inhibited, for at least some of the melt season.

Meltwater channels can therefore help to determine the location of the ice margin, and also determine the nature and style of glaciation.

From Bickerdike et al., 2018a. Meltwater channels in the Monadhleith mountains, Scotland.


Using the instructions above, allow the students to explore, interact with, and get used to the map. Encourage them to use all the functions and investigate the glacial landforms themselves.

Does anyone live near to any glacial landforms? Has anyone been to anywhere on holiday that might have been glaciated during the Younger Dryas glaciation?

Encourage the students to use the popups to work out which was the biggest icefield, and which were second and third largest. How big were they? Use the Measure tool to measure the length of the icefields.

Zoom in to an area with detailed moraine ridges, like the terminal moraines in the Lake District. Turn off the icefields by unchecking “Younger_Dryas_Extent” in the Layer List. Turn on the satellite imagery and see if the students can view the landforms for themselves.

Ask the students to think about the different kinds of glacial landysystems they can find in the Younger Dryas of Britain. For example, is there evidence for a cirque landsystem? What about a glaciated valley?

What are the processes active in these different landsystems? What does this tell you about the style of glaciation?


Bickerdike, H. L., Evans, D. J. A., Ó Cofaigh, C., & Stokes, C. R. (2016). The glacial geomorphology of the Loch Lomond Stadial in Britain: a map and geographic information system resource of published evidence. Journal of Maps, 12(5), 1178–1186.

Down the PDF map from Bickerdike et al. 2016.

Bickerdike, H. L., Evans, D. J. A., Stokes, C. R., & Ó Cofaigh, C. (2018a). The glacial geomorphology of the Loch Lomond (Younger Dryas) Stadial in Britain: a review. Journal of Quaternary Science, 33(1), 1–54.

Bickerdike, H. L., Ó Cofaigh, C., Evans, D. J. A., & Stokes, C. R. (2018b). Glacial landsystems, retreat dynamics and controls on Loch Lomond Stadial (Younger Dryas) glaciation in Britain. Boreas, 47(1), 202–224.

Introduction to Glaciofluvial Landforms

“Fluvioglacial” means erosion or deposition caused by flowing meltwter, from melting glaciers, ice sheets and ice caps. Glacial meltwater is usually very rich in sediment, which increases its erosive power.

Fluvioglacial landforms include sandar (also known as outwash plains; they are braided, sediment-rich streams that drain away downslope away from a glacier), kames and kettles, meltwater channels, and eskers.

Glaciofluvial outwash plain

Glaciofluvial systems are characterised by strong changes in flow magnitude and frequency. Flow magnitudes can fluctuate strongly on a daily basis, as melt increases and decreases over day and night. It also fluctuates seasonally, in the summer (ablation) and winter (accumulation) seasons.

Glacier meltwater can flow supraglacially (on top of the glacier ice), englacially (within the glacier ice), subglacially (below the glacier ice) and proglacially (in front of, and away from, glacier ice). Surface meltwater can reach the bed by draining through the bases of crevasses and moulins.

Fig. 1
A schematic illustration of a land-terminating section of the Greenland ice sheet, highlighting the main meltwater pathways and stores in the hydrological system. Critical areas of uncertainty regarding the future evolution of the hydrological system are numbered as discussed in the text: (1) darkening of the ice sheet, (2) surface firn densification processes, (3) surface to bed connections at higher elevations, (4) cryo-hydrologic warming, (5) rates of channelisation at the ice bed interface, (6) subglacial sediments and till deformation, and (7) basal melt rates. All of these processes are also relevant to tidewater glacier systems. From Nienow et al., 2017.

These pages outline some of the key glaciofluvial landforms associated with the passage of glacial meltwater. For more information, see Glacier Hydrology.