An esker is an elongate sinuous ridge, of either simple or compound form, composed of glaciofluvial sediments and marking the former position of streams below (subglacial), within (englacial), or on the surface (supraglacial) of glaciers. Deposited in former icewalled channels, eskers are often the most prominent glaciofluvial landform in freshly deglaciated terrain. The routing of former meltwater channels in glaciers, and their association with ice-marginal configurations, is indicated by the overall form of eskers. There are four major types of esker: (1) continuous ridges (single or multiple) that document tunnel fills; (2) ice-channel fills produced by the infilling of supraglacial channels; (3) segmented ridges deposited in tunnels during pulsed glacier recession; and (4) beaded eskers consisting of successive subaqueous fans deposited in ice-contact lakes during pulsed glacier recession. The former englacial position of some eskers is indicated by the occurrence of buried glacier ice or almost complete disturbance of the stratified core. Most eskers are aligned subparallel to the direction of former glacier flow, thereby reflecting meltwater flow toward the ice margin. If the water is flowing in pressurized conduits, it can flow over topographic obstacles at the glacier bed, giving rise to undulatory long profiles. Such undulatory, or up-and-down, profiles can also be produced by the draping of englacial and supraglacial eskers onto underlying glacier beds during ice meltout. In certain circumstances, specifically where water is flowing at atmospheric pressure, meltwater may flow down the topographic slope of the glacier bed and therefore the resultant eskers may be aligned transverse to flow ("valley" or "subglacially engorged" eskers). Such eskers are normally much shorter than normal eskers and are typically produced at the glacier margin where the ice is thin.
Eskers are relatively uncommon in the smaller subpolar glaciers due probably to the lack of well-developed subglacial channel systems with access to debris-rich ice sequences. They are very common, however, in Arctic regions previously affected by ice sheet glaciation forming continuous esker networks of ten of more than 100 km in length. Eskers can pass into stretches of erosional (Nye) channels over long distances, but it is unlikely that such complex and lengthy systems were formed in a single tunnel. It is more likely that esker and tunnel segments were formed in the marginal zone of ablation during ice sheet recession. Where eskers lie within Nye channels, it is clear that bed conditions changed from erosional to depositional modes due to changing discharges.
Eskers are composed of a wide variety of sediments ranging from sorted silts, sands, gravels, and boulders to matrix-supported gravels that have undergone relatively short travel distances, usually less than 15 km. Bedding in subglacial eskers may display a range of sedimentary structures from ripples to massive and cross-bedded gravels. In contrast, the meltout of ice in englacial and supraglacial eskers may destroy all sedimentary structures. Some eskers may contain cyclic sedimentary sequences that fine-upwards. These probably document either seasonal or annual meltwater discharges in the tunnel system responsible for esker deposition. Anticlinal structures in the bedding of eskers are often interpreted as the product of slumping of the esker margins in response to ice wall melting and arched bedding as the product of simultaneous deposition of esker ridge and flanks.
David J.A. Evans
See also Glacial Deposition; Glacial Geomorphology; Kames
Aylsworth, J.M. & W.W. Shilts, Glacial Features Around the Keewatin Ice Divide, Districts of Mackenzie and Keewatin, Ottawa: Geological Survey of Canada Paper 88-4, 1989 Bannerjee, I. & B.C. McDonald, "Nature of Esker Sedimentation." In Glaciofluvial and Glaciolacustrine Sedimentation, edited by A.V. Jopling & B.C. McDonald, Tulsa, OK: SEPM Special Publication, 1975 Benn, D.I. & D.J.A. Evans, Glaciers and Glaciation, London: Arnold, 1998
Clark, P.U. & J.S. Walder, "Subglacial drainage, eskers and deforming beds beneath the Laurentide and Eurasian ice sheets." Bulletin of the Geological Society of America, 106 (1994): 304-314 Huddart, D., M.R. Bennett & N.F. Glasser, "Morphology and sedimentology of a high arctic esker system: Vegbreen, Svalbard." Boreas, 28 (1999): 253-273 Price, R.J., "Eskers near the Casement Glacier, Alaska."
Geografiska Annaler, 48 (1966): 111-125 Shreve, R.L., "Movement of water in glaciers." Journal of Glaciology, 11 (1972): 205-214
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