A fjord is a long, narrow, steep-sided inlet of the sea that fills a valley carved by a glacier: the ice gouges a U-shaped trough deeper than the sea floor beyond it, and when the ice melts, the ocean floods in. Most fjords are deeper inland than at their mouth, where a shallow rock or moraine sill marks the spot where the glacier spread out and lost its cutting power. Norway's Sognefjord runs about 205 km inland and reaches 1,308 m (4,291 ft) below sea level, Greenland's Scoresby Sund branches across some 38,000 km², and New Zealand's Milford Sound, just 15 km long, is walled by cliffs rising 1,200 m or more straight out of the water.
Jens Esmark’s 1824 Idea: Ice, Not Rivers or Rifts
The explanation now taught in every geography class began with a Danish-Norwegian mineralogist. Jens Esmark (1763–1839), who in 1814 became Norway's first professor of geology, argued in 1824 that glaciers had once been far larger and thicker than any in Norway at the time, covering much of the country and the neighbouring sea floor. He attributed erratic boulders and moraines to ice transport, and he was the first to name glaciers as a force powerful enough to have cut the Norwegian fjords.
The rival view did not vanish quickly. In 1913 the British geologist John Walter Gregory (1864–1932) published The Nature and Origin of Fiords, arguing that fjords were of tectonic origin and that glaciers had played only a negligible part. Later research rejected his conclusion, chiefly because of two features that rivers and faults cannot explain: fjord floors that sink far below the depth of the open sea beyond them, and rocky thresholds at their mouths.
Gregory was not entirely wrong about the rock, though. Geologists now accept that existing weaknesses steered the ice. In the Hardangerfjord, fractures in the Caledonian fold belt guided where the glacier cut, and the same link between fractures and fjord direction is seen around Lyngen in northern Norway. Before the ice ages, Tertiary rivers had already cut valleys into western Norway as the land was uplifted, and those valleys gave the later glaciers a ready-made route to the sea. The modern picture is a combination: rivers and fractures set the path, and ice did the deepening.
How the Sognefjord Was Overdeepened
In the standard model, a glacier settles into a pre-glacial valley with a gently sloping floor. As it moves, it plucks blocks of bedrock and grinds the rock beneath it with the debris frozen into its base, a process called abrasion. Where tributary glaciers joined the main trunk, the combined ice was thickest and fastest, so it excavated the deepest basins. The result is an overdeepened, U-shaped trough that ends abruptly at the head of the valley.
The Sognefjord, nicknamed the King of the Fjords, shows every stage of this. Its floor lies more than 1,000 m below sea level for roughly 100 km, from Rutledal to Hermansverk, and the deepest water is in the central reaches near Høyanger. Under the water, about 200 m of sediment blankets the bottom, so the bedrock the ice actually scoured sits around 1,500 m below sea level. Yet near the mouth the floor rises abruptly to a sill only about 100 m down. Out there the ice could spread into sounds and low ground among the islands, thin out, and lose its erosive force. The cliffs above the fjord match its depth, rising almost sheer to 1,000 m and more.
The side valleys tell the same story in reverse. Smaller tributary glaciers could not cut as deep as the main one, so when the ice melted, their valleys were left perched above the main trough as hanging valleys, often with waterfalls spilling from their lips. The effect continues below the waterline: the mouth of the Fjærlandsfjord branch is about 400 m deep while the main Sognefjord nearby is 1,200 m deep, and the mouth of the Ikjefjord is only 50 m deep where the main channel beside it is around 1,300 m deep.
When the Ice Left: Rebound, Sills and the Saltstraumen Current
Melting ice did more than let the sea in. The crust, relieved of the weight of ice and eroded rock, slowly sprang back up, a process called isostatic or post-glacial rebound. In some places the land rose faster than the sea. The Drammensfjorden, an arm of the outer Oslofjord, is almost cut in two by the Svelvik ridge, a sandy moraine that lay below sea level while ice covered it and now stands about 60 m above the fjord. Where a threshold ends up above sea level, the trough behind it holds a freshwater lake instead of a fjord.
Where the sill stays underwater, it controls the whole fjord. Bolstadfjorden near Voss is 160 m deep but has a threshold only 1.5 m below the surface, and fresh water from the Vosso river forms a brackish lid that blocks circulation; its deep layers are starved of oxygen and the bed is covered in organic muck. In fjords with deeper sills, such as the Sognefjord at about 100 m, offshore winter winds set up a surface current that pulls dense coastal salt water over the threshold and renews the deep basin.
Narrow sills also make some of the strongest tidal currents on Earth. At Saltstraumen, about 10 km south-east of Bodø, up to 400 million m³ of seawater forces its way through a strait 3 km long and 150 m wide every six hours as the tide tries to fill the Skjerstad Fjord, spinning up whirlpools as much as 10 m across. The current itself is only two to three thousand years old; before that, post-glacial rebound had not yet raised the land into its present shape.
Narrow Branches: The Nærøyfjord and Geirangerfjord
The most photographed fjords are not the longest but the narrowest side arms, where tributary glaciers cut slots between high plateaus. The Nærøyfjord, a branch of the Aurlandsfjord, which is itself a branch of the Sognefjord, runs about 17 km and narrows to roughly 250 m at its tightest point, while the mountains on either side climb to about 1,761 m. South of the village of Bakka the summits on each side stand only about 3 km apart.
Farther north, the Geirangerfjord is a 15 km branch of the Sunnylvsfjorden, which in turn branches off the Storfjorden in Møre og Romsdal. Its walls rise so steeply that the shore offers almost no level ground, and the waterfalls known as the Seven Sisters and the Suitor pour into it from facing cliffs. In 2005 UNESCO listed the Geirangerfjord and the Nærøyfjord together as the West Norwegian Fjords World Heritage Site, citing their full range of landforms from active Pleistocene glaciation and post-glacial uplift.
Ilulissat and Scoresby Sund: Greenland Fjords Still Being Cut
In Norway the glaciers that carved the fjords are long gone. In Greenland, the process can still be watched. The Ilulissat Icefjord runs about 40 km west from the edge of the Greenland ice sheet to Disko Bay, roughly 250 km north of the Arctic Circle. At its head, the Sermeq Kujalleq (Jakobshavn Isbræ) glacier flows 20 to 35 m a day, the most productive glacier in the Northern Hemisphere, and it calves around 20 billion tonnes of icebergs into the fjord each year. Some bergs stand up to a kilometre tall, too deep to float out, so they ground in the shallower parts of the fjord and stay stuck, sometimes for years, until pressure from the ice behind breaks them free. UNESCO listed the fjord in 2004, partly for the glacier's role in research on climate change.
On the east coast, Scoresby Sund shows how big a fjord system can become. Its main body runs about 110 km west from a mouth 29 km wide, then splits into a tree of branches, including the Nordvestfjord and the Øfjord, that together cover about 38,000 km². The longest arm reaches 340 to 350 km inland, and while the main basin is 400 to 600 m deep, some branches plunge to 1,450 m. The English explorer William Scoresby mapped it in detail in 1822.
Milford Sound / Piopiotahi and the Southern Fiords
Fjords form best where moist westerly winds are forced up over coastal mountains and drop heavy snow to feed glaciers. That is why the most fjorded coasts are western ones: Norway, the coast from Puget Sound to Alaska, south-western New Zealand and southern Chile. In New Zealand, where the older spelling fiord survives, the type example is Milford Sound / Piopiotahi in Fiordland National Park, at 12,607 km² the largest of the country's 13 national parks.
Milford Sound runs 15 km inland from the Tasman Sea, and its walls rise 1,200 m or more on both sides. Mitre Peak, a cluster of five summits that looks like a single point from the water, reaches 1,690 m with near-vertical drops into the fiord. The sealer John Grono named the inlet in 1823 after Milford Haven in his native Wales, and the dual name, with the Māori Piopiotahi, became official under a 1998 Treaty of Waitangi settlement with Ngāi Tahu.
Chile's channels hold some of the deepest fjord water anywhere: the Messier Channel near Tortel reaches 1,358 m. Behind Chile's outer islands, a skerry-sheltered passage of channels runs about 800 km north from the Strait of Magellan.
Fjards, Förden and the Freshwater Randsfjorden
The word itself comes from Old Norse fjǫrðr, a lake-like stretch of water used for crossing and ferrying, from the same Indo-European root that gave English ford, ferry and fare. Scandinavians still use it far more loosely than geographers do, which is why some of the "fjords" on a Norwegian map are not fjords in the scientific sense.
Along the Baltic, where the ice crossed low ground instead of mountains, it carved broad, shallow inlets called fjards (Swedish fjärd), with mud flats and salt marshes instead of cliffs. The Förden of Germany's Baltic coast and the fjords of eastern Jutland form a third type, glacial inlets that follow old, beheaded river channels into the nearly tideless sea. Inland, the name drifts further still: Randsfjorden, Norway's fourth-largest lake at 140.7 km², is a long, narrow freshwater lake 135 m above sea level that only gained "fjord" in its name in records from 1691. None of these is the drowned glacial trough that Esmark described. For that, the deep, cliff-walled inlets of western Norway are still the reference, and the Sognefjord and Geirangerfjord are the places to see it, before going on to compare them with the largest fjords in the world.