Drumlin

A drumlin, from the Irish word droimnín ("little ridge"), first recorded in 1833, in the classical sense 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. Assemblages of drumlins are referred to as fields or swarms; they can create a landscape which is often described as having a 'basket of eggs topography'. The low ground between two drumlins is known as a dungeon; dungeons have colder microclimates in winter from settling cold air. Drumlins occur in various shapes and sizes, including symmetrical (about the long axis), spindle, parabolic forms, and transverse asymmetrical forms. Generally, they are elongated, oval-shaped hills, with a long axis parallel to the orientation of ice flow and with an up-ice (stoss) face that is generally steeper than the down-ice (lee) face. Drumlins are typically between long and between wide. Drumlins generally have a length:width ratio of between 1.7 and 4.1 and it has been suggested that this ratio can indicate the velocity of the glacier. That is, since ice flows in laminar flow, the resistance to flow is frictional and depends on area of contact; thus, a more elongated drumlin would indicate a lower velocity and a shorter one would indicate a higher velocity. Drumlins and drumlin swarms are glacial landforms composed primarily of glacial till. They form near the margin of glacial systems, and within zones of fast flow deep within ice sheets, and are commonly found with other major glacially-formed features (including tunnel valleys, eskers, scours, and exposed bedrock erosion). Drumlins are often encountered in drumlin fields of similarly shaped, sized and oriented hills. Many Pleistocene drumlin fields are observed to occur in a fan-like distribution. The long axis of each drumlin is parallel to the direction of movement of the glacier at the time of formation. Inspection of aerial photos of these fields reveals glacier's progress through the landscape.