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A drumlin (derived from the Gaelic word druim (“rounded hill,” or “mound”) first recorded use in 1833) is an elongated whale-shaped hill formed by glacial action. Its long axis is parallel with the movement of the ice, with the blunter end facing into the glacial movement. Drumlins may be more than 45 m (150 ft) high and more than 0.8 km (½ mile) long, and 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.

There are many theories as to the exact mode of origin and plenty of controversy among geologists interested in geomorphology. Some consider them a direct formation of the ice, while a theory proposed since the 1980s by John Shaw and others postulates creation by a catastrophic flooding release of highly pressurized water flowing underneath the glacial ice. Either way, they are thought to be a waveform (similar to ripples of sand at the bottom of a stream). It is also poorly understood why drumlins form in some glaciated areas and not in others. They are often associated with ribbed moraines.

Drumlins are common in New Yorkmarker, the lower Connecticut River valley, eastern Massachusettsmarker, the Monadnock Regionmarker of New Hampshiremarker, Minnesotamarker, Wisconsinmarker, Albertamarker, Southern Ontario, Nova Scotiamarker, Polandmarker, Estoniamarker, around Lake Constancemarker north of the Alps, County Monaghanmarker, County Mayomarker, County Cavanmarker and County Fermanagh in the northern provinces of Irelandmarker, Greenlandmarker, Hindsholmmarker in Denmark, Finlandmarker and Patagonia. Those in North America are regarded as a creation of the last Wisconsin ice age.

The islands of Boston Harbor Islands National Recreation Areamarker are drumlins that became islands when sea levels rose as the glaciers melted. Clew Baymarker in Irelandmarker 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. In County Cavanmarker, Irelandmarker, there is a unique mesh of streams and rivers which feed into and out of three hundred and sixty-five lakes which rest between the Drumlins; one lake for each day of the year.

Drumlin formation has recently been observed for the first time in Antarcticamarker in the Rutford Ice Stream.A similar formation, with a more resilient (generally composed of igneous or metamorphic rock) core, is a crag.

Drumlin soil classification is variable but often consists of a thin A soil horizon and a thin Bw horizon. The C horizon is close to the surface, and may be at the surface on an eroded drumlin.

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References



Footnotes

  1. http://www.uofaweb.ualberta.ca/water//pdfs/ShawQuatInt.pdf Shaw, John, The meltwater hypothesis for subglacial bedforms, Quaternary International 90 (2002) 5–22
  2. BBC News, "Antarctic hill surprises experts", BBC, http://news.bbc.co.uk/2/hi/science/nature/6295395.stm (accessed 2007-01-28)


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