Eskers, Riverbeds with a Twist
By Paul De Schutter
The end of the last Ice Age shaped the land in numerous and distinctive ways, but one of the more remarkable features left behind by the retreating ice sheets and glaciers are Eskers.
The word Esker is derived from the the old Irish Gaelic eiscir, meaning “ridge of gravel”, and it refers to sinuous, narrow, steep-sided ridges composed of irregular stratified drift, deposited in contact with glacial ice in either an open channel or an enclosed conduit (Drewry, 1986).
Eskers are therefore glacio-fluvial landforms: ancient river beds, formed by glacial meltwater flowing in channels along zones of weakness in or beneath glaciers or ice sheets. In general, esker deposits are very similar to regular fluvial deposits. The sediments range from cobbles and boulders in high flow-rate areas, to fine grained laminated deposits in areas of low discharge, where both parallel and crossbedded lamination can be present.
But eskers also have some unique features. As the meltwater was confined by the surrounding ice, the river sediments were deposited above the surrounding terrain, and in some cases they can even be traced uphill, as subglacial esker rivers are not only driven by gravity as ordinary rivers are, but also by the hydrostatic pressure of the overlying ice.
Eskers are predominantly formed at the margin of warmbased glaciers or ice sheets - usually when these are stagnating or retreating - because they require an ample supply of meltwater, and because advancing glaciers would quickly erode previously deposited esker sediments.
Because of this, eskers formed by meltwater from the Laurentide ice sheet in North America are mostly located south of 72° N, where conditions around the end of the last Ice Age were favourable for their formation and preservation.
Subglacial eskers form when meltwater at the base of the glacier preferentially erodes the overlying ice instead of the bedrock, thus creating a tunnel at the glacier bed. The sediments carried by the meltwater are deposited along the way, causing the bed to accrete upwards. This usually happens on broad, flat surfaces, where the velocity of the esker river is slow enough to allow deposition of the sediments (although eskers also occur in some mountain glacier settings).
Some eskers may also be deposited in channels inside the glacier (englacial), or in ice-walled trenches on top of it. These sediments are then lowered to the surface upon melting of the glacier. Sometimes eskers are part of sedimentary fans or deltas at the glacier terminus.
Eskers come in different shapes and sizes. They can be straight or sinuous, and usually have a height of a few metres, but in some cases they can be as high as 50 to 80 metres. Their length varies from a few hundered metres to tens of kilometres, and their width from a few metres to up to a hundered metres and more.
Eskers can be broad and flat-topped, or have a single or multiple crest. They are often discontinuous, as sediment is deposited only in places where the meltwater slows down. Eskers generally follow the trend of their parent glacier (parallell to the ice flow).
Sometimes blocks of the overlying ice fall into the esker sediments, forming kettle-holes.
Apart from their geological interest, eskers have also long had a very practical value.
As they are mostly made up of highly porous sand and gravel and thus are well-drained, they have been used since prehistoric times by humans - as well as animals - as natural elevated roadways (a famous example is the Eiscir Riada in Ireland, still carrying the N4/N6 from Dublin to Galway in some places), allowing safe passage through surrounding bogs and wetlands.
They have also been preferred sites for graveyards (as it’s easy to dig a hole in them, and possibly because their elevated nature would bring the dead closer to the heavens), and, more recently, golf courses.
Many eskers have been quarried extensively for construction materials, driving these landforms almost to extinction in some places.