Eskers are long, winding ridges of sand and gravel, formed by meltwater rivers within or beneath ancient glaciers. These distinctive geological formations offer valuable insights into Earth’s glacial history.
Defining Eskers: Characteristics and Appearance
Eskers are serpentine, ridge-like structures found in landscapes once covered by glaciers. Their uniform shape often resembles artificial embankments like railway lines or winding roads. These ridges consist primarily of stratified sand and gravel, sometimes with cobbles or boulders. The sediment is typically sorted by grain size, reflecting the action of flowing water.
Eskers vary greatly in size, ranging from hundreds of meters to hundreds of kilometers long. They typically stand 5 to 50 meters (16 to 160 feet) high and can be 500 to 1,600 meters (160 to 1,600 feet) wide. While often continuous, eskers may also appear as disconnected segments, with gaps separating parts. Their crests can be sharp, rounded, or flat-topped, often described as knobby.
The Formation Process
Eskers form from sediment deposited by meltwater streams flowing within or beneath glaciers, typically as glaciers melt and retreat. Meltwater carves out tunnels or channels: englacial (within the ice) or subglacial (beneath the ice).
Under immense pressure from overlying ice, these meltwater streams carry a substantial sediment load, including sand, gravel, and larger debris. As water flows, it deposits heavier materials along the floor of the ice-walled tunnels, similar to a riverbed. High pressure can even allow water to flow uphill. The amount of material deposited is influenced by the rock debris concentration in the ice and sediment supply rate.
As the glacier melts and retreats, the ice walls containing these streams disappear. The accumulated sand and gravel are left behind, forming the distinct, sinuous esker ridges. This process creates a sorted and stratified deposit, unlike unsorted glacial till. Some eskers may also form when a glacier becomes stagnant, preventing material dispersal.
Global Distribution and Identification
Eskers are found in regions once covered by large ice sheets during past glaciations. These include parts of North America (e.g., Canada, Maine, Michigan, with over a thousand eskers), Scandinavia (Sweden, Finland), and the British Isles. Their presence indicates a region was shaped by glacial activity.
Eskers are identified by their characteristic long, winding, ridge-like shape, often standing out as elevated features in flat terrain. From the air, they appear as distinct, feathery or zig-zagging lines, generally parallel to ancient ice flow. Their composition of well-sorted sand and gravel, visible in cuts, further confirms their identity.
Ecological and Human Significance
Eskers are important for both ecosystems and human activities. Ecologically, their well-drained, sandy soils support unique plant communities and specialized habitats. These elevated ridges offer drier ground and serve as natural pathways for wildlife, including migrating caribou. Wolves, foxes, and grizzly bears use eskers for denning, and their slopes can provide microclimates, supporting spruce trees beyond the typical treeline.
From a human perspective, eskers have been valuable resources. Their natural elevation and stable composition make them suitable travel routes, with some roads, like Alaska’s Denali Highway, built directly on them. The sand and gravel deposits are sought after as aggregates for construction (concrete, road building). Esker formation also provides scientists insights into ancient glacier dynamics, aiding in reconstructing past climate and understanding ice sheet response to environmental changes.