Glaciers create some of the most dramatic landscapes on Earth, carving deep valleys and sharp peaks through erosion while leaving behind hills, ridges, and plains built from deposited rock and sediment. These landforms fall into two broad categories: those shaped by the grinding, plucking power of moving ice, and those formed when glaciers melt and drop the material they’ve been carrying. Together, they account for much of the terrain across northern North America, Scandinavia, Patagonia, and any mountain range that has hosted glaciers.
How Glaciers Reshape the Land
Glaciers erode in two main ways. The first is abrasion: rocks frozen into the base of a glacier act like sandpaper, grinding down the bedrock beneath. The second is plucking, where meltwater seeps into cracks in the bedrock, refreezes, and pries loose chunks of rock that the glacier then carries away. These two processes work together to carve, smooth, and steepen the terrain in predictable patterns.
Cirques, Arêtes, and Horns
A cirque is a bowl-shaped depression carved into a mountainside where a glacier originates. Snow accumulates in a sheltered hollow, compresses into ice, and gradually deepens the basin through repeated cycles of freezing and plucking at the headwall. After the glacier melts, many cirques hold small lakes called tarns.
When two cirques erode toward each other from opposite sides of a ridge, the rock between them narrows into a thin, jagged crest called an arête. These ridgelines often look like serrated knife blades, with steep sides and low points called cols that once acted as spillways for overflowing ice. The Knife Edge on Mount Katahdin in Maine is a well-known example.
A horn forms when three or more glaciers erode a single peak from different sides. The result is a steep, somewhat pyramidal summit with flat faces and sharp edges. The Matterhorn in the Alps is the textbook case, but horns appear in glaciated mountain ranges worldwide.
U-Shaped Valleys and Fjords
Rivers cut narrow, V-shaped valleys. Glaciers transform them. As a valley glacier flows downhill, it erodes not just the bottom but the entire width of the valley floor, widening it and steepening its walls. When the ice retreats, what remains is a broad, flat-bottomed, steep-walled U-shaped valley. Yosemite Valley in California is a classic example. Before glaciation, the valley was roughly 2,000 to 2,400 feet deep as measured from its present rim. Glaciers deepened and widened it dramatically, producing the sheer cliffs of El Capitan and Glacier Point that draw millions of visitors today.
When a U-shaped valley extends to the coast and the sea fills it, the result is a fjord. Norway’s coastline is defined by these long, narrow inlets, some over a thousand feet deep. Fjords also line the coasts of New Zealand, Chile, and Alaska.
Hanging Valleys and Waterfalls
A smaller tributary glacier often joins a larger valley glacier the same way a stream feeds into a river. But the smaller glacier doesn’t erode as deeply. When both glaciers melt, the floor of the smaller valley sits far above the main valley floor. This is a hanging valley, and water flowing through it typically plunges over the edge as a waterfall. Bridalveil Fall in Yosemite drops from a hanging valley into the main valley below.
Roches Moutonnées
These are asymmetric bedrock hills shaped by a glacier passing over them. The upstream side is gently sloped, smooth, and polished from abrasion. The downstream side is steep, rough, and jagged where the glacier plucked rock away. Because of this consistent shape, geologists use roches moutonnées to determine which direction ancient ice flowed. The ridge between the smooth and jagged slopes runs perpendicular to the flow direction.
Moraines: Ridges of Glacial Debris
Glaciers carry enormous amounts of rock, from fine clay to house-sized boulders. When the ice melts, this material gets dropped in place. The specific type of moraine depends on where the debris accumulates.
- Lateral moraines form along the edges of a glacier. As ice melts in the lower portion of the glacier (the ablation zone), it drops sharp-crested ridges of rock and debris along the valley walls.
- Medial moraines appear where two glaciers merge. Rock that falls from the ridge between them ends up as a dark stripe running down the center of the combined glacier. These are mostly surface features and rarely survive intact after the ice melts.
- Terminal moraines mark the farthest point a glacier reached. They form a ridge of debris across the valley at the glacier’s snout, like a bulldozer pile at the end of a construction site.
- Ground moraines are broad, gently rolling sheets of debris left beneath the glacier as it retreats, often creating the undulating farmland seen across the upper Midwest.
Glacial Till vs. Outwash
The material glaciers leave behind comes in two distinct forms. Till is deposited directly by ice. It is completely unsorted, meaning clay, sand, gravel, and boulders are all mixed together with no layering. Outwash is deposited by meltwater streams flowing away from the glacier. Because water sorts sediment by size and weight, outwash is layered and organized, consisting mainly of rounded sand and gravel arranged in distinct horizontal bands.
This difference matters beyond geology classrooms. Till tends to hold water poorly and creates uneven terrain, while outwash plains are characteristically flat and made of layered sand and gravel. Many communities across the northern United States and Canada sit on outwash deposits, which serve as important groundwater aquifers because of how easily water moves through the sorted sediment.
Drumlins
Drumlins are smooth, elongated hills shaped like an inverted spoon, with a steeper end pointing upstream and a tapered end pointing in the direction the glacier flowed. They form beneath a glacier, though their exact origin is still debated. Leading explanations include reshaping of a deforming layer of sediment under the ice, buildup around resistant patches on the glacier bed, and even subglacial floods. Drumlins commonly appear in groups called swarms. The area around Boston and the drumlins of upstate New York are well-known examples.
Eskers and Kames
Eskers are long, sinuous ridges of sand and gravel deposited by meltwater rivers that once flowed through tunnels inside or beneath a glacier. When the ice melts away, the river’s sediment remains as a winding ridge that can stretch for miles across otherwise flat terrain. They come in several forms, from continuous tunnel fillings to beaded chains of mounds that formed where water pooled at the glacier’s edge.
Kames are irregular mounds of sand and gravel deposited by meltwater in depressions on or against stagnant ice. As the ice supporting or surrounding the deposit melts, the sediment collapses into a lumpy hill. Kames often appear alongside kettle lakes in landscapes where a glacier stalled and slowly wasted away.
Kettle Lakes
When a retreating glacier leaves behind buried or partially buried blocks of ice within its outwash deposits, those ice blocks eventually melt. The ground above collapses into the void, forming a depression that fills with water. These are kettle lakes, and they dot the landscapes of Minnesota, Wisconsin, and much of Canada. Walden Pond in Massachusetts is a kettle lake. They range from small ponds to lakes over a mile across, depending on the size of the original ice block.
Glacial Erratics
Erratics are boulders that a glacier picked up and carried far from their source before dropping them as the ice melted. They often stand out dramatically from the local bedrock, sometimes perched on ridgelines or sitting alone in open fields. Some erratics have been transported hundreds of kilometers. In Tierra del Fuego at the southern tip of South America, erratic boulder trains were carried supraglacially (on top of the glacier) from the Cordillera Darwin mountain range, likely after massive rock avalanches dumped material onto the ice surface.
Erratics are more than curiosities. By matching the rock type of an erratic to its source outcrop, geologists can reconstruct the path and extent of ancient ice sheets across entire continents.