How Fjords Form: Glaciers, Valleys, and the Sea

A glacier is a large mass of ice that moves slowly under its own weight. As it flows downhill, it can erode the landscape by plucking blocks of rock and grinding the valley floor with debris frozen into the ice. Over long periods, this erosion widens and deepens an existing river valley.

River valleys often have a V-shaped cross section because flowing water cuts most strongly into the channel bottom. Glacial valleys are typically broader and more U-shaped because thick ice presses against both the floor and sides. In mountainous coastal regions, a glacier can cut the valley floor below present sea level, especially where the ice is thick and erosion is intense.

This deep carving is the key to the whole process. A fjord is not simply any narrow sea inlet between mountains. Its shape records a glacial history, with a deep trough, steep sides, and often smaller hanging valleys where tributary glaciers once joined the main ice stream.

When climate warms, a glacier may melt and retreat up its valley. The rock basin it excavated remains behind. If the valley connects with the ocean, seawater can enter and fill the low ground. The result is a long marine inlet extending inland along the old glacial trough.

Many fjords are surprisingly deep. The greatest erosion often occurred where glacier ice was thickest farther inside the valley. Near the mouth, erosion could be weaker, or sediment and rock may form a shallower threshold called a sill. That means the fjord floor can be much deeper inland than at the point where it opens to the sea.

Sea level also matters. During ice ages, large amounts of water were stored in continental ice sheets, lowering global sea level. After the ice melted, sea level rose. At the same time, land that had been pressed down by heavy ice could slowly rebound upward. The coastline you see today reflects both glacial carving and later changes in land and water level.

Fjords share several clues that point back to glaciation:

• •Long, narrow seawater inlets extend far inland.
• •Steep mountain or rock walls rise directly from the water.
• •The underwater valley can be very deep compared with the fjord mouth.
• •Hanging valleys and waterfalls may appear along the sides.
• •Rounded or polished rock surfaces can preserve evidence of past glacier movement.

Waterfalls are common along fjord walls because smaller side valleys can end high above the main valley floor. These hanging valleys formed where tributary glaciers eroded less deeply than the main glacier. After the ice disappeared, streams flowing through the smaller valleys had to drop sharply to reach the fjord.

Remembering the process is easier if you picture the sequence rather than the final scenery: river valley, advancing glacier, deep U-shaped trough, retreating ice, then seawater flooding the hollow. That order connects the landform with the process that created it.

Fjords appear in the places you would expect from their history: mountainous coasts that were covered by thick glaciers during recent ice ages. The best known examples are in Norway, but there are also many in Greenland, Iceland, Alaska, British Columbia, southern Chile and the south-west of New Zealand. Names can be misleading. Milford Sound in New Zealand is a classic fjord despite being called a sound, a word usually used for an inlet flooded by the sea without glacial carving. Norway's Sognefjord stretches for around 200 kilometres inland and is more than a kilometre deep in places.

The shallow sill at a fjord's mouth has an important effect on the water inside. Dense, cold water can sit in the deep inner basin, trapped below the level of the sill, while fresher water from rivers and melting snow flows out across the surface. Because that deep layer may mix only occasionally with the open sea, its oxygen can be used up by decaying material and become very low. Every so often, dense seawater from outside spills over the sill and renews it. Scientists study these basins as natural laboratories for how low-oxygen waters behave.

Fjords are easily confused with rias, another kind of drowned valley, but the two record different histories. A ria forms when rising sea level floods an ordinary river valley that was never deepened by ice. Because it began as a river valley, a ria usually has a V-shaped cross section, gentler slopes and a floor that deepens steadily towards the sea, the opposite of a fjord with its deep inner basin and shallow sill. Rias often branch like the tributaries of the original river. Good examples include several estuaries in Devon and Cornwall and the rias of Galicia in north-west Spain.

Britain has fjords too. Many of the long sea lochs of western Scotland, such as Loch Etive, were carved by glaciers and have the deep basins and shallow entrances typical of fjords. The word fjord comes from Norwegian, from an Old Norse word for an inlet. Viking settlers brought a related word to Scotland, where it became firth, as in the Firth of Forth, although many firths are broad estuaries rather than true glacial fjords. The ordinary English word ford, a shallow place to cross a river, comes from the same ancient root, which carried the idea of passing through or travelling across.

The steep sides that make fjords so striking can also make them dangerous. While a glacier filled the valley, the ice helped support the rock walls. Once it melted, those walls lost that support, and centuries of water freezing and thawing in cracks can slowly loosen huge slabs. When a large rockslide plunges into a narrow fjord, it displaces water that has nowhere to spread, raising waves far taller than ordinary sea waves. In the twentieth century, rockslides at Loen and Tafjord in western Norway sent waves into villages along the shore and killed dozens of people.

The most extreme known example came in Lituya Bay, a fjord in Alaska, in 1958. An earthquake shook loose a mass of rock that fell into the head of the bay, and the resulting wave surged up the opposite slope, stripping away forest to a height of about 524 metres. Norway now monitors several unstable mountainsides continuously. At Åkneset, above a branch of the fjord system near Geiranger, sensors track the movement of a large cracked slab of rock, linked to warning systems that would give nearby communities time to move to higher ground if it began to slide.

How fjords form is a story of ice first and seawater second. Glaciers widen and deepen mountain valleys, sometimes cutting them below sea level, then retreat leaves space for the ocean to flood the trough. Follow that sequence and the dramatic shape of a fjord becomes a readable piece of glacial geography.