drumlin

Drowned drumlin in Clew Bay, County Mayo

Drumlin at Withrow Moraine and Jameson Lake Drumlin Field National Natural Landmark, Washington state

Drumlin in Cato, New York

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.^[1] 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 York, the lower Connecticut River valley, eastern Massachusetts, the Monadnock Region of New Hampshire, Minnesota, Wisconsin, Alberta, Southern Ontario, Nova Scotia, Poland, Estonia, around Lake Constance north of the Alps, County Monaghan, County Mayo, County Cavan and County Fermanagh in the northern provinces of Ireland, Greenland, Hindsholm in Denmark, Finland 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 Area are drumlins that became islands when sea levels rose as the glaciers melted. 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. In County Cavan, Ireland, 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 Antarctica in the Rutford Ice Stream.^[2]

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.

See also

• Landform
• Glacial landforms
• Lincoln Hills
• Ribbed moraines
• Roche moutonnée
• Sediment

References

• Tarbuck, Edward J.; Frederick K. Lutgens (2002). Earth: An Introduction to Physical Geography. Upper Saddle River, New Jersey: Prentice Hall. pp. 347–350. ISBN 0-13-092025-8. 
• Trenhaile, Alan (2007). Geomorphology: A Canadian Perspective. Don Mills, Ontario: Oxford University Press. pp. 183–6. ISBN 0-19-542474-3. 

Footnotes

External links

Wikimedia Commons has media related to: Drumlin

• Diagrams of an idealized drumlin
• Drumlin field, northwestern Manitoba, image from Geological Survey of Canada Canadian Landscapes Photo Collection