How Glaciers Carve and Build Landscapes

Glaciers form where more snow accumulates than melts over many years. Layers of snow compress under their own weight, becoming dense grains and eventually glacial ice. Once the ice is thick enough, gravity causes it to deform and move.

Some movement happens within the ice as crystals slowly change shape. A glacier may also slide over its bed when meltwater reduces friction beneath it. The centre and upper parts often move faster than the edges, where rock and valley walls create resistance. Gravity supplies the overall pull, but temperature, ice thickness, slope and the amount of meltwater all influence how the glacier responds.

Glacier speed varies widely. Some move only a short distance in a year, while others can move much faster during brief surges. Even slow ice can transform land because it keeps pressing, scraping and transporting material over long periods.

Water can seep into cracks in rock beneath or beside a glacier and freeze. As the glacier moves, it can pull loosened blocks away. This process is called plucking. The captured rocks become part of the moving ice.

Those rocks scrape across the ground like rough tools, grinding and polishing the surface. This abrasion can leave scratches called striations that show the direction of past ice movement. Fine ground rock may be carried away as sediment.

River valleys are often narrow and V-shaped because flowing water cuts most strongly into the bottom. Glaciers widen and deepen the whole valley, producing a broad U-shaped profile. They can also carve bowl-shaped hollows, steepen mountain peaks and deepen basins that later fill with lakes. Along coastlines, deep glacial valleys can later fill with seawater and become fjords. Smaller side glaciers may leave hanging valleys above the deeper main valley.

A glacier carries sediment ranging from fine clay to enormous boulders. When the ice melts, it drops this material. Unsorted deposits are called till, while meltwater streams sort sediment by size as they carry it away. Moraines can form along the sides, centre or front of a glacier, recording former ice positions. Meltwater may also build broad plains of sand and gravel beyond the ice.

Common clues of former glaciation include several features. No single feature proves that a glacier was present, but several clues together can provide a strong explanation.

• •U-shaped valleys with steep sides.
• •Striations scratched into exposed rock.
• •Moraines made from ridges of deposited debris.
• •Erratics carried far from their original rock source.
• •Lakes occupying glacially carved basins.

Glaciers do not only carve the ground. Their sheer weight pushes it down. An ice sheet a few kilometres thick presses the Earth's crust into the hotter, slowly flowing rock beneath, much as a heavy person sinks into a mattress. When the ice melts, the load is removed, but the crust springs back only very slowly because the rock underneath has to flow back into place. This process is called glacial isostatic adjustment.

Many places are still rising today, thousands of years after their ice sheets vanished. Parts of Scandinavia and the area around Hudson Bay in Canada are rising by millimetres each year, and northern Britain is rising while parts of southern England are slowly sinking. One visible clue is the raised beach, an old shoreline with wave cut platforms and shingle now sitting well above the present sea. These landforms record how the land has lifted since the ice left.

Two centuries ago, many naturalists explained scattered boulders and layers of loose gravel across northern Europe as the leftovers of a great flood. Some suggested icebergs drifting in floodwaters had dropped the boulders. The deposits were therefore called drift, and the word is still used for glacial sediments in Britain today. The explanation seemed reasonable because people could not imagine ice covering lowland countries, and huge boulders lying far from any rock of the same kind needed some powerful means of transport.

Observers in the Alps, including local mountain guides, had noticed that modern glaciers left polished rock, scratches and moraines, and that similar features lay far beyond the present ice. In the 1830s the Swiss-born naturalist Louis Agassiz championed the idea that vast ice sheets had once covered much of the northern continents. The proposal met strong resistance at first, but the evidence kept fitting. Striations pointed in consistent directions, erratics could be traced back to their source rocks, and moraines marked old ice edges. By the later nineteenth century, an ice age had become the accepted explanation.

Some of the most useful glacial landforms are low and easy to overlook. Drumlins are smooth, oval hills moulded beneath moving ice, often found in large groups that give the ground a look sometimes described as a basket of eggs. Each one tends to have a steeper, blunter end facing the direction the ice came from and a gentler tail pointing the way it went, so a field of drumlins works like a set of arrows. In Clew Bay in western Ireland, rising sea level has drowned a drumlin field, and the hills now poke up as a crowd of small islands.

Eskers record the meltwater instead. These long, winding ridges of sorted sand and gravel formed in rivers that flowed through tunnels inside or beneath the ice, and when the icy walls melted, the old riverbed was left standing as a ridge. The word comes from the Irish eiscir, and in parts of Ireland eskers once carried routes across boggy land. Kettle holes form when a block of ice is stranded and buried in meltwater sediment. As it slowly melts, the ground above collapses into a hollow, which often fills with water to become a small, round lake.

Glaciers can also reshape land suddenly, through floods of their own meltwater. Near the end of the last ice age, a lobe of the ice sheet covering western Canada pushed south and blocked a river valley, and water backed up into the valleys of what is now western Montana. The result was glacial Lake Missoula, a vast lake held in place by a wall of ice. Because ice floats, a deepening lake could lift and break the dam, emptying in a matter of days. The ice then advanced again, the lake refilled and the cycle repeated, probably dozens of times.

The floods tore across eastern Washington, stripping away soil and cutting into the hard basalt beneath. They left the Channeled Scablands, a maze of dry channels, giant gravel ripples and abandoned waterfalls such as Dry Falls, a cliff several times wider than Niagara. In the 1920s the geologist J Harlen Bretz argued that only a catastrophic flood could explain this landscape. Many geologists rejected the idea because it clashed with the slow, steady processes they preferred. Later work identified Lake Missoula as the water source, and aerial photographs revealed the ripples, so Bretz was eventually vindicated.

Glaciers shape land by moving, eroding and depositing material. Plucking and abrasion carve valleys and basins, while melting ice leaves ridges, boulders and sediment behind. Read those landforms together, and you can reconstruct the slow journey of ice across a landscape.