Drumlins and Other Classic Glacial Landforms
Glaciers leave behind a landscape. Long after the ice has retreated, the imprint of glaciation persists in the topography — in the shape of valleys, the distribution of sediment, the orientation of hills and ridges, and the alignment of lakes and rivers. Reading a glaciated landscape is one of the most satisfying exercises in field geology: once you know what to look for, an ordinary-seeming valley, moorland, or coastal plain reveals its history in the geometry of its hills and the character of its soils. The vocabulary of glacial landforms is extensive, but a handful of classic forms appear repeatedly across formerly glaciated terrain and are worth understanding in detail.
Drumlins
Drumlins are elongated, oval hills composed of glacial till — the unsorted mixture of clay, sand, gravel, and boulders that a glacier deposits at its base. They are typically one to several kilometres long, tens to hundreds of metres wide, and tens of metres high, with a steeper upstream face (the stoss end) and a more gently tapering downstream end (the lee end). The long axis is aligned with the direction of former ice flow, making drumlins useful indicators of palaeoglacial flow directions.
How drumlins form remains one of the genuinely contested questions of glaciology and sedimentology. The leading hypotheses involve subglacial deformation of saturated sediment, in which the glacier's base creates rhythmic instabilities in the deforming bed that build into elongated mounds — a process analogous to sand dunes forming in a river current. Other hypotheses involve erosion of pre-existing sediment mounds by ice flow rather than deposition. The difficulty is that we cannot directly observe subglacial processes during active glaciation; all our knowledge comes from relict features and theoretical modelling.
Drumlin fields are common across much of Ireland, Scotland, northern England, Wisconsin and Minnesota in the United States, and southern Ontario in Canada. The drumlin fields of County Down in Northern Ireland and County Cavan in Ireland are among the best examples in the world — the landscape of low, green, rounded hills packed together is a reliable indicator of lowland glaciation. In New York State, drumlins around the Finger Lakes — elongated by former ice flow — are visible from the road.
Eskers
Eskers are long, narrow, sinuous ridges of sand and gravel deposited by meltwater streams flowing in tunnels beneath a glacier or ice sheet. As the glacier retreats, the channel deposits are left behind as a raised ridge that winds across the landscape, sometimes for tens or hundreds of kilometres, following the former course of subglacial rivers.
The Esker Riada in Ireland runs from County Galway to Dublin and formed beneath the last ice sheet; it served for millennia as an elevated causeway across the boggy Irish midlands. The eskers of Finland and Sweden are similarly long and prominent, and many have been used as road routes since prehistoric times. Canadian Shield eskers in northern Ontario, Quebec, and the Northwest Territories are among the largest known, extending hundreds of kilometres and providing elevated, well-drained terrain in otherwise flat boreal landscape.
Eskers are valuable as records of subglacial drainage — their position and orientation reveal where meltwater was concentrated beneath the ice sheet, which in turn reflects the pressure gradients and routing of basal meltwater. They also have economic value: esker gravels are a major source of aggregate for construction in regions like Finland and Ireland where alternative sources of coarse sediment are limited.
Kames and Kame Terraces
Kames are irregular mounds and ridges of stratified sand and gravel deposited by meltwater at the glacier margin — in crevasses, in ponds at the ice edge, or against valley walls where the glacier pressed against adjacent slopes. Kame terraces are flat-topped benches running along valley sides, deposited where meltwater streams flowed between the glacier surface and the valley wall.
Kames are particularly common in areas where stagnant ice melted in place rather than retreating as a flowing mass — the meltwater deposits were simply dropped where the ice was. The resulting landscape is hummocky, with mounds of various sizes interspersed with kettle holes where buried ice blocks finally melted.
Kettle Holes and Kettle Lakes
Kettle holes are depressions in glacial outwash plains and moraines created when blocks of ice buried or surrounded by sediment melted after the main ice retreat. The volume of the melting ice block creates a corresponding volume of subsidence, forming a circular or oval depression. Many kettle holes fill with water and become kettle lakes — rounded, relatively shallow lakes with no inlet or outlet stream.
The lake district of northeastern Poland, the Mecklenburg Lake Plateau in northeastern Germany, and the lake-dotted landscapes of Minnesota and Wisconsin in the United States all owe much of their character to kettling. Walden Pond in Massachusetts, famously associated with Henry David Thoreau, is a kettle lake.
Outwash Plains and Braided Rivers
Beyond the terminal moraine of a glacier, meltwater carrying heavy sediment loads spreads across a broad, low-gradient plain, depositing sand and gravel in a network of shifting braided channels. This outwash plain (or sandur in Icelandic, a term widely adopted in glacial geology) develops as streams repeatedly shift their course, abandoning old channels and cutting new ones, building up a flat expanse of coarse sediment.
Iceland's southern coast is dominated by vast sandur, some covering hundreds of square kilometres, formed by outwash from the Vatnajokull ice sheet. During jokulhlaup — glacial outburst floods — these sandur can be inundated by enormous water flows that rapidly rework and extend the deposits.
The glacier map can guide you to glaciated regions worldwide where these classic landforms are best preserved and most accessible. From the drumlins of Ireland to the eskers of Finland and the sandur of Iceland, the glacial legacy is written clearly in the landscape for those who know how to read it.