A drumlin, from the Irish word droimnín ("little ridge"), first recorded in 1833, is an elongated hill in the shape of an inverted spoon or half-buried egg formed by glacial ice acting on underlying unconsolidated till or ground moraine.
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 are formed a short distance within the receding glacier ice and record the final direction of ice movement. Drumlins occur in symmetric, spindle, parabolic, and transverse asymmetrical forms. 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).
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. However the recent retreat of a marginal outlet glacier of Hofsjökull in Iceland provided the opportunity for direct study of a drumlin field with formation of more than 50 drumlins ranging from 90–320 m (300–1,050 ft) in length, 30–105 m (98–344 ft) in width, and 5–10 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. 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.
A drumlin's long axis is parallel with the movement of the ice; it is roughly symmetrical around the long axis. Drumlins are typically 1–2 km (0.62–1.2 mi) long, less than 50 m (160 ft) high and between 300 to 600 metres (980–2,000 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.
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.
Examples of drumlins
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.
The literature also documents extensive drumlin fields in England, Scotland and Wales, Switzerland, Poland, Estonia (Vooremaa), Sweden, around Lake Constance north of the Alps, County Monaghan, County Mayo and County Cavan in the Republic of Ireland, County Fermanagh and County Down in Northern Ireland, Germany, Hindsholm in Denmark, Finland and Greenland.
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.
Drumlins are common in Upstate New York (between the south shore of Lake Ontario and Cayuga Lake) , the lower Connecticut River valley, eastern Massachusetts, the Monadnock Region of New Hampshire, Michigan (central and southern Lower Peninsula), Minnesota, 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, Douro-Dummer, Ontario, the Thelon Plan of the Northwest Territories, Alberta, Nunavut and Nova Scotia. 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.
In 2007 drumlins were observed to be forming beneath the ice of a West Antarctica glacier.
- Crag and tail, a similar formation, with a more resilient core (generally composed of igneous or metamorphic rock)
- Glacial landforms
- Lincoln Hills
- Ribbed moraines
- Roche moutonnée
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- A satellite image of the region of Hofsjökull where drumlin growth has been observed (see ). The drumlins can be observed between pools of water.
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|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
- Word of the day defines drumlin.