A drumlin is an elongated, streamlined hill sculpted beneath a massive glacier or ice sheet. The term comes from the Gaelic word druimnÃn, meaning “little ridge.” Drumlins are formed by the immense pressure and movement of glacial ice acting on the loose sediment, or till, at the glacier’s base. Their shape and orientation provide geologists with a direct map of the direction that ancient ice flowed, offering insight into the dynamics of past ice sheets.
Defining the Physical Characteristics of Drumlins
A drumlin is characterized by its distinctive, asymmetrical, and streamlined shape, often described as resembling a half-buried egg or an inverted spoon. The long axis of the hill is always aligned parallel to the direction of the former ice flow, which is its defining geometric feature. Drumlins are typically between 250 and 1,000 meters long, 120 to 300 meters wide, and can reach up to 50 meters in height.
The hill’s asymmetry is defined by its two ends: the stoss end and the lee end. The stoss end, which faced the advancing glacier, is steeper and blunter, bearing the initial force of the ice movement. Conversely, the lee end, which is on the down-ice side, is smoother and tapers gradually, creating a streamlined “tail.” The typical length-to-width ratio of a classic drumlin falls between 1.7-to-1 and 4.1-to-1, indicating the hill was intensely molded by the overriding ice.
The Glacial Process of Drumlin Formation
The exact mechanism of drumlin formation remains a complex and actively debated topic in glacial geology. Two primary, often overlapping, theories attempt to explain how the moving ice creates these distinct landforms. The Depositional Theory suggests that drumlins form by the accumulation of sediment beneath a moving glacier when the ice experiences changes in its ability to carry debris, such as a localized drop in velocity. The material is deposited around an initial obstacle, and the continuous flow of the ice streamlines this accumulating sediment into the characteristic oval shape.
The Erosional Theory posits that drumlins are residual features created by the massive ice sheet carving away pre-existing sediment. In this model, the glacier erodes and reshapes an older layer of glacial sediment, leaving behind the streamlined hills. The immense pressure and friction at the base of the ice sheet can deform the underlying sediment, a process sometimes called the “deforming bed model.” This subglacial deformation molds the softer sediment into the streamlined forms. Some theories also involve catastrophic subglacial meltwater floods, which could erode the bed or deposit sediment in scours.
Internal Composition and Material Makeup
The material that makes up a drumlin provides clues about its formation, though the composition can be highly varied. Most drumlins are composed primarily of glacial till, a poorly sorted mixture of sediment. This till contains a chaotic blend of clay, silt, sand, gravel, and boulders, deposited directly by the ice without the sorting action of water. The long axes of the particles within the till often align with the drumlin’s long axis, reinforcing the idea of formation under directional ice flow.
Some drumlins are classified as “rock-cored drumlins” because they contain a nucleus of solid bedrock or older, more resistant sediment. The ice sheet sculpted the surrounding, softer material while the hard core acted as a protective anchor, shielding the till deposited around it. The presence of rock cores supports the erosional and streamlining theories, as the ice was forced to flow over and around the obstruction. The internal structure can also show layers of sorted, water-deposited sediment, suggesting meltwater activity played a role before the final streamlining by the ice.
Identifying Drumlins and Global Distribution
Drumlins are rarely found in isolation and almost always occur in large, dense groupings known as drumlin fields or swarms. A single field can contain hundreds or even thousands of these hills clustered together. The resulting landscape is often described as having a characteristic “basket of eggs” topography due to the rounded, parallel appearance of the hills. These swarms are found in formerly glaciated regions across the globe, indicating the widespread nature of Pleistocene ice sheets.
Notable drumlin fields are extensive and can be observed in places like central New York and Wisconsin, Ireland, Canada, and parts of Europe, including Sweden and Finland. For geologists, the uniform orientation of drumlins within a swarm is their most significant feature. By mapping the long axes, scientists can accurately reconstruct the direction and extent of the ancient ice flow. The size and elongation of the hills also offer insights into the velocity of the glacier, with more elongated forms suggesting faster ice movement.