In glaciated regions across the Northern Hemisphere — Sweden, Finland, Canada, the northern United States, Ireland — you can find long, narrow, winding ridges rising out of otherwise flat terrain. They are typically composed of sand and gravel rather than the surrounding bedrock or clay till, and they often have a serpentine, snaking pattern that does not match the local drainage or topography. These ridges are eskers — the fossilised paths of rivers that once flowed within or beneath the great ice sheets of the last ice age. Eskers preserve a record of how meltwater moved through and beneath the ice, and they tell us about both the dynamics of vanished glaciers and the geography of present-day landscapes shaped by glaciation. Eskers are among the more distinctive features of glaciated landscapes and an important resource for the sand and gravel industry, but they are also subtle features that require trained eyes to recognise. Understanding eskers is understanding one of the more curious aspects of how ice sheets work.
The Name and the Concept
"Esker" comes from the Irish word "eiscir" meaning ridge — a reflection of the prominence of these features in Ireland's glacial landscape. The Esker Riada, a major prehistoric east-west route across central Ireland, follows the line of a series of eskers and gives the landform its name. Eskers are formed by rivers of meltwater flowing within or beneath glaciers, depositing sand and gravel as the water slows or the flow regime changes. When the surrounding ice retreats, the sand and gravel deposit is left behind as a ridge — an "ice-contact" landform that records the location of the now-vanished water channel.
How They Form
Meltwater is constantly produced in active glaciers and ice sheets — from surface melting in summer, from basal melting due to geothermal heat and frictional heating, and from various other sources. The meltwater flows through the ice in complex networks of channels — surface channels (supraglacial), tunnels within the ice (englacial), and tunnels at the ice-rock interface (subglacial). Subglacial channels are particularly important for esker formation. These channels can be tens of metres in diameter and can extend for hundreds of kilometres beneath major ice sheets. The flowing water carries sand and gravel along the channel bed. As the water flow speed varies (slowing in straight sections, accelerating in narrowed ones), it deposits some of its load. When the surrounding ice eventually melts away, the deposited sand and gravel are left as a ridge marking the position of the former channel.
The Geometry
Eskers typically appear as long, narrow, sinuous ridges, often a few hundred metres wide and several metres to tens of metres tall. Some eskers are short — just a few hundred metres long. Others are remarkable — the Munro Esker in Ontario extends for over 250 kilometres. The "Sallachan Esker" in Scotland extends for about 9 kilometres. Eskers can be more or less continuous (single-ridge systems extending for tens of kilometres) or fragmentary (a series of short ridges with gaps between them). The fragmentary character suggests that water flow in the subglacial channel was sometimes interrupted or modified. The crest of an esker is typically narrow and sharp, with steep sides falling away to the surrounding terrain.
The Composition
Eskers are composed of sand and gravel — sedimentary material that was transported and sorted by flowing water. The grain size depends on the energy of the flow that deposited the material; eskers can range from coarse cobble-and-pebble gravel (deposited by fast-flowing water) to fine sand (deposited by slower flow). The deposits are typically rounded (the grains have been transported and worn smooth) and well-sorted (the grain sizes are similar in a given layer, indicating a single deposition event with characteristic flow speed). This sand-and-gravel composition contrasts with the surrounding glacial till, which is typically unsorted (containing grains of all sizes, from clay to boulders), deposited directly by melting ice without water sorting. The contrast in composition makes eskers stand out geologically and visually.
Eskers and the Sand-and-Gravel Industry
Eskers are economically important as sources of high-quality sand and gravel — a building material in essentially every country with a glaciated history. The naturally sorted character of esker material means it requires less processing than poorly-sorted alluvial gravels or crushed bedrock. Major sand and gravel operations in Finland, Sweden, Ontario, Quebec, the northern US, Ireland, and many other places have historically focused on eskers. The mining process essentially removes the ridge — extracting the sand and gravel for construction use. The environmental tradeoff is significant: eskers are non-renewable resources that took thousands of years to form, and their removal alters local hydrology, ecology, and landscape. Many countries now restrict esker mining to preserve at least some eskers in their natural state.
Eskers and Drumlins
Eskers are often associated with other glacial landforms, particularly drumlins. A drumlin is a streamlined hill, elongated parallel to the direction of ice flow, that forms beneath active glaciers. Drumlins are typically composed of glacial till rather than sand and gravel. Drumlin fields in places like central New York, Wisconsin, Minnesota, Ireland, and Scotland often have eskers cutting through them — the eskers reflect the position of subglacial water channels, while the drumlins reflect the streamlined deposition of till beneath the ice. The combination tells the story of how ice and water moved together at the base of the glacier. Other related landforms include kames (mounds of sand and gravel deposited at the margin of glaciers) and outwash plains (broader sand-and-gravel deposits laid down by meltwater rivers in front of glaciers).
Where Eskers Are Found
Eskers are particularly prominent in regions that were under continental ice sheets during the last glacial maximum — northern Europe (Finland, Sweden, Estonia, parts of Norway, the British Isles), Russia, Canada, and the northern United States. Finland has perhaps the most spectacular esker landscape, with extensive esker fields running for hundreds of kilometres across the country. The Salpausselkä esker system in southern Finland is a particularly famous example. Sweden's Brunkebergsåsen and Stockholm's underground topography are both essentially built on eskers. Ireland's eskers, including the Esker Riada, have been important to the country's geography and culture since prehistoric times. Canada's Munro Esker in Ontario and the Thelon River esker complex in Nunavut are among the longest known. The northern US has substantial esker development in New England, the Great Lakes region, and the northern Plains.
Eskers as Roads
Eskers have historically been important as natural transportation corridors. Their elevated, well-drained, sand-and-gravel surface makes them ideal for paths and roads through otherwise marshy or rough country. The Esker Riada in Ireland was a major east-west route across the country for over a thousand years, used by everyone from Iron Age travellers to medieval Irish kings to modern road builders. Similar esker-based routes exist in Finland, Sweden, and the northern US. In some cases, modern paved roads and railways still follow esker crests for kilometres at a time, taking advantage of the natural ridge as a foundation. The Stockholm metro's blue line was partly constructed by tunnelling through esker material because it was easier to excavate than the surrounding bedrock.
Eskers and Ecology
Eskers create distinctive ecological habitats. The sandy, well-drained soils on esker crests support different plant communities than the surrounding clayey, poorly-drained soils on the adjacent flat terrain. In areas where the surrounding landscape is peatland or wetland, the dry esker crests serve as ecological "islands" supporting pine forests or other dry-adapted ecosystems. The contrast in vegetation often makes eskers visually distinctive on aerial photographs — a green ribbon of trees winding through brown peat. Esker habitats support various specialised species that depend on the sandy, dry conditions. Some endangered plants in Finland are esker specialists, depending on the unique conditions that eskers provide.
Eskers in Climate History
Eskers preserve a record of glacial dynamics that helps reconstruct the last ice age. The orientation and length of eskers indicate the direction of meltwater flow beneath the ice sheet. The size of eskers indicates the volume of water that flowed through the subglacial channels. The timing of esker formation (determined by dating the surrounding sediments) helps reconstruct when the various parts of an ice sheet were melting. These records are important for understanding how the great ice sheets of the last glaciation responded to climate forcing and for predicting how modern ice sheets in Greenland and Antarctica might respond to current and future climate change. The Laurentide Ice Sheet of North America and the Scandinavian Ice Sheet of Europe each produced enormous amounts of meltwater during their retreats, and the eskers they left behind are direct evidence of that process.
Modern Glaciers and Subglacial Hydrology
The processes that formed eskers in the past are still active today beneath modern glaciers and ice sheets. Subglacial hydrology — the network of channels and water bodies beneath active glaciers — is a major research area in glaciology. Subglacial lakes have been discovered beneath the Antarctic ice sheet, including the famous Lake Vostok (about the size of Lake Ontario, buried under 4 kilometres of ice). Subglacial channels and floods have been observed in Iceland (where they sometimes produce sudden, catastrophic floods called jökulhlaups when subglacial lakes drain). Greenland's ice sheet has a substantial network of subglacial water that may be affecting ice flow speeds. Understanding modern subglacial hydrology informs our understanding of how past eskers formed, and the past eskers provide constraints on our models of modern subglacial systems. The connection between past and present subglacial dynamics is one of the active frontiers of glaciological research.