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.
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).
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.
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.
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.
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
