A fjord is a long, narrow, steep-walled sea inlet carved by a glacier that cut its valley far below sea level and was then flooded by the ocean when the ice melted. Fjords form only where thick valley glaciers once reached the coast, so they cluster in Norway, Greenland, Alaska and British Columbia, Chile, New Zealand's Fiordland, Iceland and Scotland.
In everyday Norwegian, fjord means almost any inlet. Geographers use it more narrowly: a fjord is defined by how it was made, and that origin explains its depth, its walls and its waterfalls.
How a Glacier Digs Below Sea Level
Most fjords began as ordinary river valleys running from mountains to the sea. During the Pleistocene glaciations, which ended roughly 11,700 years ago, those valleys filled with ice hundreds to more than a thousand metres thick. A glacier plucks loose fractured bedrock and scours the floor with debris frozen into its base. Because the ice fills the whole valley, it widens the sides as well as the floor, turning a V-shaped river valley into a broad U-shaped trough with near-vertical walls.
The key difference from river erosion is that ice is not tied to sea level. A river cannot cut much below the level of the water it drains into, but a glacier heavy enough to stay grounded keeps grinding as it flows into the sea. Where several tributary glaciers converged, the ice was thickest and fastest, and the bedrock was dug deepest. Geologists call this overdeepening: the inner basin of a fjord is often far deeper than the open sea outside it.
Near the coast the glacier spread out, thinned and lost its erosive power, leaving a shallower rock lip, often topped with a terminal moraine. This threshold or sill at the fjord mouth is the opposite of what a river valley shows. A drowned river valley deepens steadily seaward; a fjord is typically deepest well inland, behind a sill that can be only a fraction as deep.
Smaller side glaciers carried less ice and cut their valleys less deeply, so when the main glacier melted those tributaries were left stranded high on the trough walls as hanging valleys. Their streams now pour off the lip as waterfalls, which is why fjord walls are lined with long, thin cascades. Finally, the melting ice sheets raised global sea level by more than 100 metres, and the sea flooded the troughs to give the fjords seen today.
Norway: Sognefjord, Geirangerfjord and a Coast Far Longer Than It Looks
Western Norway has the classic fjord landscape, and it set the template for the scientific term. Sognefjord runs about 205 kilometres inland from the North Sea coast north of Bergen, making it the longest fjord in Norway and the second longest in the world. Its deepest point reaches about 1,300 metres below sea level, deeper than much of the adjacent North Sea continental shelf, while the sill near its mouth is only a small fraction of that depth.
Further south, Hardangerfjord extends roughly 180 kilometres. To the north, the much shorter Geirangerfjord (about 15 kilometres) and Nærøyfjord, a branch of Sognefjord around 17 kilometres long and in places only about 250 metres wide, were inscribed together on the UNESCO World Heritage List in 2005 as the West Norwegian Fjords. Their walls rise up to 1,400 metres straight from the water. The Seven Sisters waterfall on Geirangerfjord's northern wall drops from a hanging valley in a series of parallel streams, a textbook illustration of how the trough cut off its tributaries.
Fjords also explain why Norway's coastline figures look so odd. The straight-line length of the mainland coast is roughly 2,500 kilometres, but the Norwegian Mapping Authority measures the mainland coast including every fjord at close to 29,000 kilometres, and over 100,000 kilometres once the islands are added. Farms sat on the narrow deltas at fjord heads, and boats were the main transport until tunnels and ferries arrived in the twentieth century. More on that landscape is in our guide to Norway's geography.
Greenland, Svalbard and the Arctic Fjords
The world's longest fjord system is in East Greenland. Scoresby Sund (Kangertittivaq) reaches roughly 350 kilometres inland from the Greenland Sea when its longest branch, Nordvestfjord, is included. Unlike Norway's fjords, many Greenland fjords are still active: outlet glaciers of the Greenland Ice Sheet calve icebergs directly into them, so the process that carved Sognefjord can be watched in progress. Ilulissat Icefjord, a UNESCO site since 2004, is fed by one of the fastest-moving glaciers in the world.
Svalbard, Iceland's Westfjords, the Faroe Islands and the coasts of Baffin and Ellesmere islands in Arctic Canada are also deeply cut by fjords. Fjords appear on mountainous mid- and high-latitude coasts where heavy snowfall once fed large glaciers. The countries with the most glaciers today line up closely with the countries that have fjords.
British Columbia, Alaska and Newfoundland's Landlocked Fjord
The Pacific coast of North America, from Puget Sound in Washington to the Alaska Panhandle, is one of the most fjord-rich coastlines on Earth. The Coast Mountains of British Columbia were buried by the Cordilleran Ice Sheet, and dozens of long inlets now cut into them. Howe Sound (Átl'ka7tsem in the Squamish language), just north-west of Vancouver, is about 42 kilometres long and is often called North America's southernmost fjord, though Puget Sound has a similar glacial origin. Further north, Knight Inlet and Bute Inlet each run more than 70 kilometres into the mountains. Alaska adds Glacier Bay, Tracy Arm and Kenai Fjords, where tidewater glaciers still reach the sea.
On the opposite side of the continent, Western Brook Pond in Gros Morne National Park, Newfoundland, shows what can happen after the ice leaves. It is a glacier-carved trough about 16 kilometres long, flanked by cliffs roughly 600 metres high, but it holds fresh water. When the ice sheet melted, the land, relieved of its weight, slowly rose in a process called isostatic rebound, lifting the fjord mouth above sea level and cutting it off from the Gulf of St Lawrence. Geologists call such features landlocked fjords. Gros Morne became a UNESCO site in 1987.
Fiordland and Chile's Patagonian Channels
In the Southern Hemisphere, the south-west corner of New Zealand's South Island holds 14 fiords (the local spelling) along a coast of granite and gneiss. They are called "sounds" because early European mapmakers took them for river valleys. Milford Sound, or Piopiotahi, is about 15 kilometres long, with Mitre Peak rising 1,692 metres almost directly from the water. Doubtful Sound, roughly 40 kilometres long, is deeper and more remote. Fiordland National Park, about 12,500 square kilometres, is part of the Te Wāhipounamu World Heritage Area listed in 1990. Our Fiordland guide covers the park itself.
Fiordland receives several metres of rain a year, and that rain creates an unusual marine environment. Fresh water stained brown by forest tannins floats on top of the denser seawater, blocking light. As a result, deep-water species such as black coral grow at depths of only a few metres to a few tens of metres, far shallower than in the open ocean.
Across the Pacific, southern Chile has the largest fjord region in the Southern Hemisphere. South of Puerto Montt, from the Aysén region to Tierra del Fuego, the Andes meet the sea in a maze of channels, islands and fjords left by the Patagonian Ice Sheet. The Northern and Southern Patagonian Ice Fields are its remnants, and glaciers such as Pío XI and San Rafael still discharge ice into fjords and lagoons. Much of this coast is reachable only by boat.
Fjord vs Ria vs Firth
Several coastal inlets look alike on a map; the difference lies in how each was made.
- Fjord: a glacial trough flooded by the sea. It has a U-shaped cross-section, steep walls, hanging valleys, and usually a shallow sill at the mouth with deeper water inland.
- Ria: a river valley drowned by rising sea level, with no glacial carving. It has a V-shaped or gently sloping cross-section, a branching tree-like outline, and gets steadily deeper toward the sea. The Rías Baixas of Galicia in Spain gave the landform its name; Sydney Harbour and the estuaries of south-west England are other examples.
- Firth: a Scots word for a coastal inlet or estuary, a name rather than a single process. The Firth of Forth and Moray Firth are broad estuaries, not fjords, although some firths do have glacial origins.
- Sea loch: the Scottish name for an inlet that is often a true fjord. Loch Etive and Loch Nevis on the west coast have the classic trough and sill. Some freshwater lochs are fjords cut off from the sea; Loch Morar, at about 310 metres, is the deepest lake in the British Isles.
A quick field test is to look at the depth profile. If the water gets deeper inland behind a shallow lip, a glacier made it. If it shallows evenly toward the head, a river did. A related comparison, between ice-cut fjords and river-cut gorges, is in our fjords vs canyons article.
What the Sill Does to the Water
The sill does more than mark the end of the ice. Because it is shallow, it limits exchange between the deep basin and the open ocean. Rivers and meltwater add a layer of fresher, lighter water at the surface, while the deep basin can hold old, dense seawater for months or years. In fjords with shallow sills and little mixing, the bottom water can lose most of its oxygen, and sediments there preserve fine annual layers that record past climate. A pulse of dense ocean water spilling over the sill eventually flushes the basin.
The same sheltered, layered water supports rich ecosystems. Norway's Trondheimsfjord has cold-water Lophelia coral reefs at unusually shallow depths, and British Columbia's glass sponge reefs, several of them in Howe Sound, were thought extinct until living ones were found in 1987. Sheltered fjord water is also why Norway, Chile and Canada farm salmon in fjords.
To test how well you can place Sognefjord, Scoresby Sund and Milford Sound on a map, try the world geography quiz.