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 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 York, the lower Connecticut River valley, eastern Massachusetts, the Monadnock Region of New Hampshire, Minnesota, Wisconsin, Southern Ontario, Nova Scotia, Poland, Estonia, around Lake Constance north of the Alps, Ireland, 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.
Drumlin formation has recently been observed for the first time in Antarctica in the Rutford Ice Stream. A similar formation, with a more resilient (generally composed of igneous or metamorphic rock) core, is a crag.