A fjord (or fiord) is a long, narrow estuary with steep sides, made when a glacial valley is flooded by the sea. The seeds of a fjord are laid when a glacier cuts a U-shaped valley through abrasion of the surrounding bedrock by the rocks and sediment it carries. Many such valleys were formed during recent ice ages when the sea was at a much lower level than it is today. At the end of such a period, the climate warms up again and glaciers retreat. Sea level rises due to an influx of water from melting ice sheets and glaciers around the world (it rose over 100 m after the last ice age), inundating the vacated valleys with seawater to form fjords.
Fjords are often very deep in their upper and middle reaches, in the case of Norway's Hardangerfjord dropping 800 m (2,624 ft) below sea level, although fjords generally have a sill or rise at their mouth associated with the previous glacier's terminal moraine.