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A drumlin (Irish droimnín, a little hill ridge) 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 150 ft (45 m) high and more than ½ miles (0.8 km) 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 to ribbed moraines. Drumlins are common in New York, the lower Connecticut River valley, eastern Massachusetts, Minnesota, Wisconsin, Southern Ontario, Poland, Ireland, Finland and Patagonia. They are regarded as a creation of the last Wisconsin ice age. 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. A similar formation, with a more resilient (generally composed of igneous or metamorphic rock) core, is a crag.
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