A landform created when a glacial valley is flooded by the sea is called a fjord. Fjords are long, narrow sea inlets with steep sides or cliffs, formed in valleys that glaciers carved into the underlying rock.
As a glacier moves downhill, it erodes a valley through abrasion and plucking. Unlike a river, which usually creates a V-shaped valley, a glacier can widen and deepen the valley into a broad U-shaped trough. When the climate warms and the glacier retreats, seawater may occupy the overdeepened valley, creating a fjord.
Fjords are often deeper than the nearby ocean because glaciers can excavate the valley floor below sea level. Many also have a shallow sill near their mouths, where the glacier eroded less forcefully or deposited sediment. This can restrict water circulation and produce distinctive tidal currents.
A common mix-up is confusing a fjord with a ria. A ria is a drowned river valley, while a fjord is specifically associated with glacial erosion. Norway is famous for fjords, but major examples also occur in Chile, New Zealand, Alaska, Greenland, Iceland, and Antarctica.