Landforms of fluvioglacial deposition: eskers and outwash plains

Overview

Outwash plain and kettle hole.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is an esker? Describe it.

   Model answer

   An esker is a long, narrow, sinuous ridge of sorted and stratified sand and gravel, winding across the landscape rather like a railway embankment or an abandoned road. It is typically 5 to 30 metres high with steep sides and a narrow crest, and it may be traced for anything from a few hundred metres to tens of kilometres, sometimes with tributary branches joining it. It runs broadly parallel to the direction of former ice flow. Its material is water-laid: rounded pebbles and clean sand in clear beds, with little clay, so it drains freely. Ireland has some of the best-developed esker systems in the world, in the central lowland.

2. 2

   How does an esker form?

   Model answer

   It is deposited by a meltwater stream flowing in a tunnel at the base of, or within, a glacier. That stream carries a very heavy load of sand and gravel supplied from the glacier bed. As discharge falls, particularly towards the end of the melt season and during general deglaciation when the ice thins and the tunnel enlarges, the stream loses velocity and deposits its bedload on the floor of the tunnel, building up a channel fill. When the surrounding ice finally melts away, the confining walls disappear and the sediment is let down onto the ground surface, standing up as a ridge in the shape of the vanished tunnel. Eskers are therefore essentially fossil casts of subglacial river channels.

3. 3

   Why can an esker run uphill?

   Model answer

   Because the water that deposited it was under pressure rather than flowing freely under gravity. A stream in a tunnel beneath hundreds of metres of ice is confined, and its flow is driven by the hydrostatic pressure of the overlying ice, which depends on the slope of the ice surface rather than the slope of the ground. Water in such a system will happily flow up an adverse gradient in the bed provided the ice surface continues to fall in that direction. The result is that eskers cross valleys and ride over low hills in a way no ordinary river deposit could, and this ability to run against the slope of the land is one of the clearest pieces of evidence that they formed subglacially.

4. 4

   What material is an esker made of, and how is it arranged?

   Model answer

   Chiefly sand and gravel, with rounded to sub-rounded clasts, very little clay or silt, and a high permeability. The material is sorted, so any one bed is of broadly uniform grain size, and it is stratified, with clear layers reflecting the fluctuating discharge of the meltwater stream that laid it down: coarse gravel beds record high-discharge episodes and finer sand beds record lower flows. Beds frequently show cross-bedding produced by migrating bedforms in the channel, and the sides of the ridge are often disturbed, faulted or slumped, because the sediment collapsed outwards when the ice walls that had supported it melted away.

5. 5

   Why does an esker run roughly parallel to former ice flow, and how might that lead to confusion with a drumlin?

   Model answer

   Because the subglacial tunnel that formed it drained water from the interior of the ice sheet towards the margin, which is broadly the same direction the ice itself was moving. Since a drumlin is also elongated parallel to ice flow, the two can share the same alignment in the same landscape, and both appear as elongated hills. They are easily separated on closer inspection. A drumlin is broad, low, smooth and oval, made of unsorted clay-rich till, and is one of hundreds of similar mounds; an esker is narrow, steep-sided, sharp-crested, winding rather than streamlined, made of sorted and stratified sand and gravel, and continues for a far greater distance relative to its width.

6. 6

   What is an outwash plain?

   Model answer

   An outwash plain, or sandur, is a broad, gently sloping apron of sorted and stratified sand and gravel spread out across the lowland beyond the margin of a glacier or ice sheet. It is a fluvioglacial depositional landform, built entirely by meltwater rather than by ice. Its surface is nearly flat, falling very gently away from the ice front, and it is crossed by a network of shallow, shifting braided channels. It may extend for many kilometres and reach tens of metres in thickness. Because the material is clean, permeable sand and gravel, an outwash plain drains freely and often carries a productive aquifer beneath it.

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