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World Geography25 Essential Exam Concepts
What Is a Fjord and How Is It Formed? Glacial U-Shaped Valleys & Sills
A fjord (or fiord) is a long, narrow, steep-sided marine inlet carved into mountainous coastal terrain by glacial activity, characterized by towering vertical cliffs and dramatic water depths that frequently exceed the depth of the adjacent open sea. Derived from the Old Norse word "fjörðr" (meaning "where one fares across" or a passageway), fjords represent some of the most striking erosional landforms produced by glacial action on Earth. They are distributed primarily along high-latitude coastlines where mountainous topography intersected with extensive ice sheets during the Pleistocene epoch, notably in Norway, Greenland, Canada, Alaska, Chile, and New Zealand.
The formation of a fjord begins during continental and alpine glacial periods when massive valley glaciers advance seaward. Driven by gravity and immense hydrostatic pressure, the glacier erodes the underlying bedrock through two dominant physical processes: plucking (the quarrying and lifting of fractured bedrock blocks frozen into the ice base) and abrasion (the grinding of the bedrock floor by rock fragments embedded in the sliding glacier). Unlike river systems, which erode riverbeds into V-shaped valleys, the massive lateral and vertical cutting power of a glacier transforms pre-existing fluvial valleys into broad, steep-sided U-shaped glacial troughs.
Crucially, glaciers possess the physical capability to erode bedrock far below contemporary sea level through a phenomenon termed glacial overdeepening. Near the seaward mouth of the valley, as the glacier enters the sea, the ice becomes buoyant and thins, dramatically reducing its erosive energy. In addition, the glacier deposits terminal moraines of unsorted till at this boundary. Consequently, the mouth of a fjord features a shallow submerged bedrock ridge or morainic threshold known as a sill.
Following global warming and deglaciation, eustatic sea levels rose, flooding these carved U-shaped valleys with marine seawater. The shallow sill restricts deep oceanic water exchange, creating unique, highly stratified hydrological environments with low-salinity surface waters fed by glacial meltwater overlying dense, oxygen-depleted marine bottom waters.