Esker in the NWT of Canada
Eskers are mined for gravel
Eskers
Esker: A long narrow ridge, often sinuous, composed of stratified sediment and marking the former location of a glacial tunnel.
The esker (Ir. Eiscir)
is one of the most striking landforms of fluvioglacial deposition. In
low-lying, boggy terrain these snaking ridges dominate the terrain, providing
vantage points and dry route ways. On valley floors, the eskers lie within
former ice margins marked by moraine systems, indicating that these ridges
form beneath glaciers. Eskers are aligned parallel to former ice flow. In a
valley, an esker will be orientated down-valley and this is one way to
distinguish this landform from any moraine ridges running across the valley
floor.
Eskers vary in shape and size. Most are sinuous, with a height of a few to
several tens of metres. The longest continue for several km but most are
shorter or discontinuous. Eskers can be broad and flat-topped, or have a
single crest or split into parallel ridges. A beaded esker has broad
hillocks strung out at intervals along its length. Eskers may locally rise
up-slope, a clear indication of water moving under pressure in a pipe-like
conduit.
Eskers are usually formed of washed sand and gravel. The free-draining ridges
are easily quarried, making eskers attractive targets for aggregate
extraction. The sediments are usually horizontally- and cross-bedded sands and
gravels but vary widely, a reflection of variations in flow regimes and in
sediment supply.
Eskers are casts of sub-glacial tunnels or ice-walled canyons near to an ice
margin. Meltwater in glaciers may collect to flow through networks of large
tunnels and the water is often highly charged with debris. Where and when
velocity falls then sediment may be laid down on the floor of the tunnel. If a
tunnel becomes blocked by sediment or ice fall then water and sediment is
ponded behind the blockage. If the tunnel is developed on the base of the ice
then the sediments in the esker will be preserved with little disturbance. In
contrast, if the tunnel lies within the ice then the sediments will be
considerably disturbed as the ice melts and the thread of sediment is let down
on the valley floor.
Eskers are discontinuous because sedimentation may not take place along the
entire length of the sub-glacial tunnel. Blocks of the overlying ice may fall
into the tunnel, later melting to give kettle-holes along the esker.
Alternatively, an esker ridge may become segmented by erosion, either by
meltwater during deglaciation or by the action of post-glacial rivers.
Eskers are predominantly formed at the margin of warm-based glaciers or ice
sheets during ice retreat or stagnation. The accumulation of the esker
requires a great deal of sediment and a high volume of meltwater flow,
conditions met as glaciers melt.
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.
Beaded eskers are deposited in segments, with each
segment ending in a bead where a delta formed in a proglacial lake.
Often eskers form only one
element in a complex system of ice-marginal landforms.