The glacier’s upper surface is never simple. Meltwater gathers below the equilibrium line in streams, pools in lakes, and descends through moulins into the depths. Beneath the ice it may flow in pipe-like conduits or in thin sheets, and a change between those modes can trigger surging. Some glaciers advance in sudden bursts, then fall back into slower motion, as if they have remembered a faster tempo. Temporary rates of 90 metres per day have been recorded when pressure and heat allowed the base to melt and water to collect below the ice. In Greenland, where glaciers can move faster than one kilometre per year, glacial earthquakes follow, with seismic magnitudes as high as 6.1.

The pace of a glacier is often measured in a single number, but that number hides a world of difference. Mean speed is commonly around one metre a day, though stagnant patches may barely move at all, while Jacobshavn Isbræ in Greenland has reached 20 to 30 metres a day. Speed depends on slope, thickness, snowfall, confinement, bed hardness, and the amount of meltwater. When the ice thickens, it insulates its base, stores more heat, and increases the driving stress. When it thins, it cools and slows. The glacier is always negotiating with its own weight, and with the land beneath it.

As it moves, the glacier carves. Plucking lifts blocks from fractured bedrock as water freezes in cracks and expands. Abrasion grinds the rock smooth, turning fragments into rock flour, fine enough to tint meltwater pale and milky. Striations and chatter marks preserve the direction of movement, like handwriting cut into stone. In the valleys of the Alps, the Andes, and the Canadian north, these marks reveal where the ice went long after the ice itself has gone. Glacial erosion deepens hollows and exaggerates pre-existing lows, and in fjords it can cut a kilometre down, steering the ice into channels that later become sea-filled trenches.

The landforms left behind are a second life of the glacier. Moraines mark the edges and terminal positions of ice, lateral ridges along the sides, medial ridges where tributary glaciers meet, and ground moraine spread beneath the old ice. The word moraine came from French peasants describing the embankments at the glacier’s edge in the Alps, and it has since become one of geology’s most useful names. Drumlins, those canoe-shaped hills, point to the direction of the ice that moulded them, and in some places, such as east of Rochester, New York, there are around 10,000 of them in a single field.

Before glaciation, a river valley is usually a V-shape. Under ice, it becomes U-shaped, deepened and widened into a trough. Spurs are truncated, hanging valleys are left above the main floor, and cirques gather at the heads of valleys where the ice first forms. If the valley runs to the sea, it can become a fjord. Two cirques back to back may sharpen a ridge into an arête, and several cirques around one summit may leave a pyramidal peak, a horn. The glacier does not merely pass through the mountains. It edits them.

Roches moutonnées show another signature of this passage. Their up-glacier faces are smoothed by abrasion, while their down-glacier faces are torn by plucking, leaving a rounded, asymmetrical knoll, like a sheep’s back polished by a giant hand. Outwash from the melting ice spreads beyond the terminus, laying down stratified sand and gravel in valley trains and plains, while kettles form where buried blocks of ice melt and leave round hollows. Eskers, long sinuous ridges of sand and gravel, trace the paths of subglacial streams that once ran in tunnels beneath the ice.

Some of the finest material the glacier makes is carried far by the wind. Rock flour lifted from bare ice becomes loess, a deep blanket of silt that can settle hundreds of metres thick, as in parts of China and the Midwestern United States. Katabatic winds help to sweep it from the glacier’s surface. In this way, the glacier reaches beyond its own valley, scattering its work over continents. What begins as snow in a cirque ends as dust on a distant plain.

For all their grandeur, glaciers are sensitive instruments. Their mass changes with precipitation, mean temperature, and cloud cover, and that makes them among the clearest indicators of climate change. Since the end of the Little Ice Age around 1850, glaciers around the world have retreated substantially. There was a slight cooling between 1950 and 1985 that let many alpine glaciers advance, but after 1985 retreat and mass loss became larger and increasingly widespread. The World Glacier Monitoring Service has tracked reference glaciers, and they have lost ice every year since 1988.

That retreat matters far beyond the mountains. Glacial ice is the largest reservoir of fresh water on Earth, and together with the ice sheets it contains about 69% of the world’s freshwater. In temperate and seasonal climates, glaciers store water as ice in the cold season and release it later as meltwater, feeding rivers when other sources are scarce. In high-altitude and Antarctic environments, the seasonal difference may not be enough to produce much meltwater at all. Where the timing is right, the glacier is not only a sculptor but a reservoir, a patient supplier of life.

The geography of glacier country is uneven and precise. Extensive ice survives in Antarctica, Argentina, Chile, Canada, Pakistan, Alaska, Greenland, and Iceland. Mountain glaciers spread through the Andes, Himalayas, Rockies, Caucasus, Scandinavian Mountains, and Alps. Snezhnika glacier in Bulgaria, at 41°46′09″ N, is the southernmost glacial mass in Europe. New Guinea’s small glaciers on Puncak Jaya are shrinking rapidly. Africa holds ice on Mount Kilimanjaro, Mount Kenya, and in the Rwenzori Mountains. Even oceanic islands such as Svalbard, Jan Mayen, New Zealand, Heard, Bouvet, and Kerguelen have their own glacial histories.

But the glacier is also a creature of absence. In parts of the Arctic, such as Banks Island, and in the McMurdo Dry Valleys of Antarctica, the cold is so dry that glaciers cannot form. The same is true in much of lowland Siberia, Manchuria, and central and northern Alaska during the Quaternary, where the air was too dry to feed the snowpack. In the Atacama-adjacent peaks of Bolivia, Chile, and Argentina, mountains rise high enough, from 4,500 to 6,900 metres, yet precipitation is too scarce for snow to accumulate into ice. The glacier depends on both cold and supply, and one without the other is only weather.

Human industry has now entered the story in force. Rising carbon dioxide and other greenhouse gases have warmed the planet, and human influence is the principal driver of the cryosphere’s present changes. The ice responds through ice-albedo feedback: as it melts, darker ground is exposed, absorbs more sunlight, and warms still further. A study of the Alps from 1995 to 2022 found that glaciers there accelerate and slow down together across great distances, showing how tightly their motion is bound to climate. In 2023, Switzerland lost 4% of its glacier volume, and the country now spends about US $500 million a year on barriers, avalanche nets, and drainage to defend Alpine settlements.

Sea level rises as meltwater runs off, and the IPCC treats this as a slow-onset event with grave consequences. Coastal settlements and infrastructure are encroached upon, small islands and low-lying coasts face existential threat, groundwater becomes saline, and storms, floods, and subsidence compound the damage. The loss is not abstract. A glacier that has been steady for centuries can, within a few decades, become a source of risk for cities and harbours far away. Its retreat is measured in metres, but its consequence is measured in nations.

Scientists read this history in ice cores. Glaciers can be hundreds of thousands of years old, and their layered ice preserves bubbles of ancient air. By melting or crushing samples from progressively deeper layers, researchers reconstruct earlier climates and compare the gases trapped within. Those measurements confirm that for at least the last million years, global temperature has moved with carbon dioxide concentration. The glacier is therefore not only a landscape but an archive, one written in snowfall and sealed by pressure.

When the great masses of ice press on the Earth, they depress the crust into the mantle by as much as a third of their thickness. Then, after the ice melts, the crust rises again in post-glacial rebound. This is happening now in Scandinavia and the Great Lakes region of North America. On a smaller scale, recently deglaciated ground in Iceland and Cumbria can crack in dilation-faulting as the compressed rock springs back. Even the land remembers the glacier after the glacier is gone.

The glacier’s reach extends beyond Earth as well. On Mars, the polar ice caps show glacial deposits, and at mid-latitudes there are lobate debris aprons that conceal ice beneath rocky covers. On Pluto, in 2015, New Horizons found Sputnik Planitia, a basin of nitrogen ice with polygonal cells and glacial flows at its margins. The forms differ, but the logic is familiar: ice under gravity, moving slowly, shaping its world. The glacier, then, is not merely a feature of mountain weather or polar cold. It is a way matter behaves when time, weight, and temperature are allowed to speak together.

And so the story closes where it began, with motion disguised as stillness. A glacier can be a valley glacier, an ice cap, an ice shelf, a tidewater glacier that calves bergs into the sea, or one of the two great ice sheets of Antarctica and Greenland. It can be temperate, polar, subpolar, cold-based, warm-based, or polythermal. It can creak with crevasses, surge for a season, or retreat for a century. Yet every form begins with snow that outlasts summer, and every one ends by leaving a mark. In the world’s coldest places, the final fact is simple: the ice is never truly still.