Geologically, a fjord (ˈ or ; also spelt fiord) is a long, narrow inlet

Inlet

An inlet is a narrow body of water between islands or leading inland from a larger body of water, often leading to an enclosed body of water, such as a sound, bay, lagoon or marsh. In sea coasts an inlet usually refers to the actual connection between a bay and the ocean and is often called an...

 with steep sides or cliffs, created in a valley carved by glacial activity

Glacier

A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

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A fjord is formed when a glacier cuts a U-shaped valley by abrasion

Abrasion (geology)

Abrasion is the mechanical scraping of a rock surface by friction between rocks and moving particles during their transport by wind, glacier, waves, gravity, running water or erosion. After friction, the moving particles dislodge loose and weak debris from the side of the rock...

 of the surrounding bedrock. Glacial melting is accompanied by rebound of Earth's crust as the ice load and eroded sediment is removed (also called isostasy

Isostasy

Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic...

 or glacial rebound). In some cases this rebound is faster than sea level rise.