Britannica Sources used in this answer

How do fjords form from glaciated valleys?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Snow-capped, rugged Atlas Mountains encircle a green valley with scattered trees, terraced fields, and a few small buildings in Morocco. — valley The rugged Atlas Mountains surround a valley in Morocco.

Fjords are formed through the marine inundation of glaciated valleys following the melting of massive glaciers.12

The process begins when thick glaciers move through existing river valleys, deepening and widening them into characteristic U-shaped valleys because this shape offers the least frictional resistance to the ice. These glaciers are so heavy and powerful that they can erode the valley floor to depths far below sea level.13

As the climate warms and glaciers melt, the sea level rises, causing marine waters to flood these deep, steep-walled troughs. The resulting fjord often features a submerged ridge or "sill" near its mouth, composed of glacial deposits, which can trap deep seawater within the basin. Because the erosive power of a glacier is greatest near its source, fjords are typically deepest in their middle and upper reaches.124