Lowland Glacial Features: Outwash Plains, Kettle Holes, and Eskers

🌊 Lowland Glacial Features: Outwash Plains, Kettle Holes, and Eskers

Beyond the terminal moraine — the ridge marking where the glacier's front (snout) was located — lies a very different landscape. Here, meltwater rivers flowing away from the glacier front spread out across a broad, flat plain, carrying and depositing sorted sediment. This is the periglacial outwash zone, and it contains some of the most geographically and practically important glacial deposits.

Outwash Plain (Sandur)

As the glacier melted, torrents of meltwater flowed away from the snout and out across the flat land in front of the glacier. These meltwater rivers were heavily loaded with sediment — sand, gravel, and silt — carried out from under and in front of the glacier. As the rivers spread out across the plain and lost energy, they dropped their sediment. The heaviest, coarsest material (gravel, pebbles) was deposited closest to the glacier front; finer material (sand, silt) was carried further away and deposited further from the ice. This creates a graded sequence of sediment from coarse near the glacier to fine at distance.

The rivers on the outwash plain were characteristically braided — they divided into multiple channels separated by bars of deposited sediment, shifting constantly as their loads changed. A flat outwash plain with braided rivers is called a sandar (plural: sandur) — this Icelandic word is now used in English-language geography.

The critical difference between outwash sediment and till: outwash is sorted and stratified (particles of similar sizes deposited in layers, with coarser material lower and finer material above). Till is unsorted and unstratified (all sizes jumbled together). This difference is used in the field to distinguish glacial from fluvioglacial deposits.

Kettle Holes — depressions left by buried ice

When a glacier retreats, large blocks of ice may break off the snout and become buried in the outwash sediment deposited around them. These blocks of dead ice can remain buried for hundreds or thousands of years, insulated by the sediment above. When they finally melt, the overlying sediment collapses into the void, leaving a bowl-shaped depression called a kettle hole. If the kettle hole is deep enough to intersect the water table, or if it receives enough rainfall, it fills with water — becoming a kettle lake. Kettle holes are typically circular or oval in plan view, with steep sides and a flat or rounded floor.

Kettle holes are common features of outwash plains. A landscape containing many kettle holes and small kettle lakes is described as pitted outwash. They are found widely across the English Midlands, East Anglia, and lowland Scotland — areas covered by the last glaciation.

Eskers — ridges of subglacial river sediment

An esker is a long, sinuous (winding) ridge of sorted gravel and sand that snakes across the landscape. Eskers look curiously out of place — a ridge of sediment that runs across hills and valleys without regard for the current drainage pattern. They can be up to 30 metres high and stretch for tens of kilometres.

Eskers form from meltwater rivers that flowed within or beneath the glacier — subglacial or englacial rivers. These rivers, running under enormous pressure in tunnels within the ice, deposited sorted gravel and sand in their beds as they flowed. When the glacier melted, the ice walls of the tunnels disappeared, but the sediment deposited by the subglacial rivers remained — left as a ridge following the former course of the subglacial drainage. The ridge is sinuous because subglacial rivers meander beneath the ice just as surface rivers meander.

Eskers are excellent sources of sand and gravel for the construction industry, and many have been quarried. They are common in Ireland, Sweden, Finland, and Canada — regions that experienced extensive glaciation.