Among all the hills that ice sheets left behind, the drumlin is the one that records which way the ice was moving. A drumlin is a smooth, elongated hill shaped like an inverted spoon or a half-buried egg, made mostly of till (the unsorted clay, sand and boulders dragged along under a glacier), and moulded beneath moving ice so that its long axis points along the direction of flow, with the steeper, blunt end facing up-ice. Most are 250 to 1,000 m long and 120 to 300 m wide, and they come in swarms: Ireland's Clew Bay has more than a hundred drowned drumlins as islands, Dodge County in Wisconsin is said to have the densest concentration in the world, and the largest fields of all lie across northern Canada, where the Laurentide Ice Sheet once stood. The name comes from the Irish droimnín, "little ridge", first recorded in 1833.
Several other glacial landforms look like drumlins from a car window: smooth bedrock knobs, castle rocks with sloping tails, long gravel ridges and ribbed moraines. Each one formed differently, and setting them side by side is the quickest way to recognise a real drumlin.
Drumlin vs Roche Moutonnée: Lembert Dome Faces the Other Way
A roche moutonnée is a knob of solid bedrock that a glacier rode over. On the up-ice side, the stoss, the ice smoothed and polished the rock by abrasion, often leaving scratches called striations. On the down-ice side, the lee, meltwater seeped into cracks, refroze onto the moving glacier and pulled away blocks, a process called plucking. The result is a gentle, polished slope facing the oncoming ice and a rough, steep cliff facing away from it.
That profile is the exact opposite of a drumlin. A drumlin's steep end faces the approaching glacier and its gentle tail trails behind; a roche moutonnée's gentle slope faces the ice and its cliff trails behind. A drumlin is also usually built of loose sediment, whereas a roche moutonnée is always carved from bedrock, an erosional feature rather than a depositional one. Smoothed bedrock bumps that lack the plucked lee face are called whalebacks or rock drumlins, and these blur the line between the two.
Lembert Dome in Yosemite National Park is a textbook case. The granite rises 800 ft (240 m) above Tuolumne Meadows and the Tuolumne River, with clear stoss and lee slopes; John Muir called it Glacier Rock. The term itself was coined in 1786 by the Alpine explorer Horace-Bénédict de Saussure, who thought the rocks resembled the wigs of his day, smoothed down with mutton fat. The usual translation, "sheep rock", is a misreading. In Sweden, much larger examples are called flyggbergs.
Drumlin vs Crag and Tail: Edinburgh’s Castle Rock and the Royal Mile
A crag and tail shares the drumlin's orientation, with its blunt, high end facing the ice, but it is built the other way round. The head is a mass of hard rock, often a volcanic plug, that the glacier could not wear down; behind it, sheltered from the ice, softer rock and sediment survived as a long ramp, the tail. A drumlin, by contrast, is usually moulded sediment throughout, sometimes around a core of rock or older till, rather than a soft tail sheltered behind an exposed crag.
Edinburgh is built on the best-known example. Castle Rock is a plug of dolerite, the hard, cooled remains of a volcanic pipe formed about 350 million years ago in the early Carboniferous. It reaches 130 m above sea level with cliffs up to 80 m high on the south, west and north. The ice flowed from the west, so the dolerite protected the softer sedimentary rock to its east, and that sheltered ridge became the Royal Mile, sloping gently down towards Holyrood. The castle can only be approached easily from the east, along the tail. Stirling Castle and North Berwick Law stand on the same kind of crag, and Salisbury Crags and Arthur's Seat form another in Edinburgh itself.
Drumlin vs Esker: The Esker Riada Between Dublin and Galway
An esker is a long, winding ridge of sand and gravel, often compared to a railway embankment. It forms when a river running through a tunnel inside or beneath a glacier drops its load on the tunnel floor. When the ice walls melt, the riverbed is left standing as a ridge. Eskers are generally 20 to 30 m high and can run for hundreds of kilometres, with gaps between their segments.
Three differences separate eskers from drumlins. First, the material: esker gravel was sorted and layered by flowing water, while drumlin till is a jumble of particle sizes. Second, the shape: an esker snakes along the route the meltwater took, whereas a drumlin is a compact oval. Third, the pattern: eskers run singly or as a branching network, while drumlins sit in fields of hundreds or thousands of near-identical hills, all aligned.
The word esker is Irish too, from eiscir, a ridge separating two plains. The Esker Riada, a chain of eskers left at the end of the last ice age about 10,000 years ago, crosses the middle of Ireland from Dublin to Galway through nine counties, roughly 200 km. Its higher, drier ground carried the Slighe Mhór, the Great Way, through the midland bogs, the N6 Dublin–Galway road still closely follows it, and in tradition it marked the line dividing Ireland in two after a battle at Mag Lena in the second century. There is more on how these ridges form in the guide to eskers.
Drumlin vs Rogen Moraine: Ridges Across the Flow at Lake Rogen
Rogen moraines, also called ribbed moraines, are the drumlin's closest relatives. Both form under the ice, both are made mostly of till, and both appear in large, regularly spaced groups. The difference is direction. A drumlin lies parallel to the ice flow; a Rogen moraine is a broad, wavy ridge lying across it, usually 10 to 30 m high, 150 to 300 m wide and 300 to 1,200 m long. From above, a field of them looks like the ribs of a washboard rather than a basket of eggs.
They take their name from Lake Rogen in Härjedalen, Sweden, the type locality, now protected in the Rogen Nature Reserve, and they are common in Fennoscandia, Scotland, Ireland and Canada, mostly in what were the central areas of the ice sheets. Drumlins often occur right beside them and may have formed at the same time. One hypothesis even links them directly: if the ice changed direction by about 90 degrees, existing drumlins and flutes could have been reshaped into transverse ridges. As with drumlins, no single explanation fits every example. For the difference between these subglacial ridges and the end moraines heaped up at an ice margin, see what a moraine is.
Clew Bay, Dodge County and Múlajökull: Drumlin Fields and How They Formed
Real drumlins are easiest to recognise in groups. At Clew Bay on the Atlantic coast of County Mayo, rising sea level after the last ice age flooded the hollows between drumlins, leaving more than a hundred small islands on the landward side of the bay, Ireland's best example of sunken drumlins. Local legend claims 365 of them, "an island for every day of the year". In Wisconsin, the Green Bay lobe of the ice sheet moulded drumlins across Dodge County, many of which now rise as islands within Horicon Marsh.
The fields reach far beyond Ireland and Wisconsin. The largest formed beneath the Laurentide Ice Sheet across Nunavut, the Northwest Territories and the northern parts of Saskatchewan, Manitoba, Ontario and Quebec, and drumlins occur in every Canadian province and territory. In the United States they cluster in central New York between Lake Ontario and Cayuga Lake, in eastern Massachusetts and in Michigan's Lower Peninsula, while in South America a large field straddles the Strait of Magellan around the airport at Punta Arenas. Within any field the shape carries information: length-to-width ratios generally fall between 1.7 and 4.1, and it has been suggested that longer, slimmer drumlins record slower ice and stubbier ones faster flow.
How the hills formed is still debated. One school holds that they are constructional: till was plastered on layer by layer beneath the ice, and the stones inside line up with the flow, which geologists test by plotting pebble orientations, a method called till fabric analysis. Another holds that they are erosional, carved from an existing soft bed, with erosion near a drumlin reaching about a metre of sediment a year. A third, more contested idea attributes them to huge floods of meltwater under the ice, citing scoured bedrock at French River, Ontario. Drumlins made entirely of granite or hard limestone suggest no one process explains them all.
Iceland offers a field caught soon after it formed. As the Múlajökull outlet glacier of Hofsjökull retreated, it exposed more than 50 drumlins, 90 to 320 m long, 30 to 105 m wide and 5 to 10 m high, spread in a fan around the ice front. Each horizontal till bed inside them records a separate surge of the glacier. In 2007, drumlins were even observed forming beneath a West Antarctic ice stream. Drumlins sit within a wider family of features that ice leaves behind, set out in the overview of glacial landforms.