Drumlin

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For the band, see Drumlin (band).
Elongate and forested drumlins south of Puerto Williams, Chile. Flow direction here was at time of formation from west to east (left to right on picture).
Drowned drumlin in Clew Bay, Ireland
Drumlin at Withrow Moraine and Jameson Lake Drumlin Field National Natural Landmark, Washington state. The glacier flowed from left to right.

A drumlin, from the Irish word droimnín ("littlest ridge"), first recorded in 1833, and in the classical sense is an elongated hill in the shape of an inverted spoon or half-buried egg[1] formed by glacial ice acting on underlying unconsolidated till or ground moraine.

Drumlin formation

Drumlins and drumlin clusters are glacial landforms, composed primarily of glacial till, which have been extensively studied. Geologists have proposed several theories about their origin. They can form both near the margin of glacial systems and within zones of fast flow deep within the ice sheet. They record the direction of ice movement at the time of formation. Drumlins occur in symmetric, spindle, parabolic, and transverse asymmetrical forms. However it is increasingly being recognised that there is no true 'drumlin' shape, and that rather subglacial bedforms can take on a wide range of shapes and sizes.[2] Drumlins are commonly found with other major glacially-formed features and are related on a regional scale to these landforms. The large-scale patterns of these features exhibit spatial organization of the drumlin-forming flows with related tunnel valleys, eskers, scours, and exposed bedrock erosion (scalloping and sichelwannen).[3]

One formation theory originally proposed in the 1980s by John Shaw and collaborators suggested that drumlin creation occurs by a catastrophic flooding release of highly pressurized water flowing underneath the glacial ice.[4] This theory includes deposition of glaciofluvial sediment in cavities scoured into the glacier bed by subglacial meltwater, and remnant ridges left behind by erosion of soft sediment or hard rock by subglacial meltwater. However this is perhaps the most widely disputed of the proposed mechanisms of formation. The more widely accepted proposed methods can be split into two camps:[5]

  1. Constructional; the landforms are the result of sediment being manipulated into shape, for example via subglacial deformation.
  2. Remnant/erosional; the landforms are part of a landscape that has been formed by erosive processes removing material from an unconsolidated bed.

The recent retreat of a marginal outlet glacier of Hofsjökull in Iceland[6] provided the opportunity for direct study of a drumlin field with formation of more than 50 drumlins ranging from 90 to 320 m (300–1,050 ft) in length, 30 to 105 m (98–344 ft) in width, and 5 to 10 m (16–33 ft) in height. This, when combined with drumlin formation identified through imaging beneath the West Antarctica ice, resulted in a significant step in geomorphologic understanding. The Hofsjökull marginal drumlins formed through a progression of subglacial depositional and erosional processes with each horizontal till bed within the drumlin created by an individual surge of the glacier.[7] Erosion under the glacier in the immediate vicinity of the drumlin can be on the order of a meter's depth of sediment per year, with the eroded sediment forming a drumlin as it is repositioned and deposited.[8]

Geomorphology

A drumlin's long axis is parallel with the movement of the ice; it is roughly symmetrical around the long axis.[9] Drumlins are typically 1 to 2 km (0.62–1.24 mi) long, less than 50 m (160 ft) high and between 300 to 600 metres (980–1,970 ft) wide. Drumlins generally have a consistent ratio of 2:3.5 width to length dimensions. Drumlins are often in drumlin fields of similarly shaped, sized and oriented hills. Drumlins usually have layers indicating that the material was repeatedly added to a core, which may be of rock or glacial till. The composition of drumlins varies depending on the area in which they are found, and can consist of similar material to the till of the surrounding moraine or be composed almost entirely of bedrock, sand and gravel or various mixtures thereof.

Many Pleistocene drumlin fields are observed to occur in a fan-like distribution.[10] The Múlajökull drumlins of Iceland are also arrayed in a splayed fan distribution around an arc of 180°.[7]

Soil development on drumlins

Drumlin soil is variable but on recently formed drumlins often consists of a thin A soil horizon (often referred to as "'topsoil'" which accumulated after formation) and a thin Bw horizon (commonly referred to as "'subsoil'"). The C horizon, which shows little evidence of being affected by soil forming processes (weathering), is close to the surface, and may be at the surface on an eroded drumlin. Below the C horizon the drumlin consists of multiple beds of till deposited by lodgment and bed deformation. On drumlins with longer exposure (e.g. in the Lake Ontario drumlin field in New York State) soil development is more advanced, for example with the formation of clay-enriched Bt horizons.[7]

Examples of drumlins

Drumlin Field in Western New York state. The drumlins align with glacial flow.

Europe

The retreat of Icelandic glacier Múlajökull, which is an outlet glacier of Hofsjökull, recently exposed a 50 drumlin cluster, which serves as the basis for improved understanding of drumlin formation.[7]

A steam train on the Downpatrick and County Down Railway in the drumlin belt of Ulster near Downpatrick in County Down, Northern Ireland.

The literature also documents extensive drumlin fields in England, Scotland and Wales,[11] Switzerland,[12] Poland, Estonia (Vooremaa), Latvia, Sweden, around Lake Constance north of the Alps, County Monaghan, County Mayo and County Cavan in the Republic of Ireland, County Fermanagh, County Armagh and County Down in Northern Ireland, Germany, Hindsholm in Denmark, Finland and Greenland.[11]

Clew Bay in Ireland is a good example of a 'drowned drumlin' landscape where the drumlins appear as islands in the sea, forming a 'basket of eggs' topography. Drumlins are typically aligned parallel to one another, usually clustered together in numbers reaching the hundreds or even thousands. These clusters can sometimes lead to the natural emergence and growth of complex water systems.

North America

Drumlins are common in Upstate New York (between the south shore of Lake Ontario and Cayuga Lake),[13] the lower Connecticut River valley, eastern Massachusetts, the Monadnock Region of New Hampshire, Michigan (central and southern Lower Peninsula),[14] Minnesota,[11] and Wisconsin. Drumlins, which are usually found in swarms or large groups, occur in every Canadian province and territory. Swarms of thousands of drumlins are found in Southern Ontario (along eastern end of Oak Ridges Moraine near Peterborough, Ontario, as well as areas to the west near Dundas, Ontario and Guelph, Ontario[15]), Central-Eastern Ontario (Douro-Dummer, Ontario), the Thelon Plan of the Northwest Territories, Alberta, southwest of Amundsen Gulf in Nunavut and West Lawrencetown, Nova Scotia.[16] The majority of those observed in North America were formed during the Wisconsin glaciation.

The islands of Boston Harbor Islands National Recreation Area are drumlins that became islands when sea levels rose as the glaciers melted.

Asia

Drumlins are found at Tiksi, Sakha Republic, Russia.[11]

South America

Extensive drumlin fields are found in Patagonia,[11] for example near Punta Arenas Carlos Ibáñez del Campo Airport and on Navarino and Gable Island in the Beagle Channel.

Antarctica

In 2007 drumlins were observed to be forming beneath the ice of a West Antarctica glacier.[17]

See also

References

  1. ↑ Menzies(1979) quoted in Benn, D.I. & Evans, D.J.A. 2003 Glaciers & Glaciation , Arnold, London (p431) ISBN 0-340-58431-9
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ A satellite image of the region of Hofsjökull where drumlin growth has been observed (see Lua error in package.lua at line 80: module 'strict' not found.). The drumlins can be observed between pools of water.
  7. ↑ 7.0 7.1 7.2 7.3 Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ M. Spagnolo, C.D. Clark, A.L.C. Hughes, P. Dunlop & C.R. Stokes (2010). The planar shape of drumlins. Sedimentary Geology 232, 119-129
  10. ↑ Patterson, C.J., and Hooke, R.L., 1995, Physical environment of drumlin formation: Journal of Glaciology, v. 41, p. 30–38.
  11. ↑ 11.0 11.1 11.2 11.3 11.4 Lua error in package.lua at line 80: module 'strict' not found.
  12. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  13. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  14. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  15. ↑ http://tcc.customer.sentex.ca/GT/OD.html
  16. ↑ Gray, Charlotte; The Museum Called Canada: 25 Rooms of Wonder, Random House, 2004 ISBN 978-0-679-31220-8
  17. ↑ Smith, A.M., Murray, T., Nicholls, K.W., Makinson, K., Aðalgeirsdóttir, G., Behar, A.E., and Vaughan, D.G., 2007, Rapid erosion, drumlin formation, and changing hydrology beneath an Antarctic ice stream: Geology, v. 35, p. 127– 130, doi:10.1130/G23036A.1.

Further reading

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External links

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