Like all fjords, the Hardanger Fjord in Norway is much deeper than a typical coastal-plain estuary, with a maximum depth of some 2,600 ft (800 m). Near its mouth is a sill just 500 ft (150 m) deep. At 114 miles (183 km) long, it is the third-longest fjord in the world. Hardanger Fjord was formed about 10,000 years ago, when a large glacier that had carved out and occupied a deep U-shaped valley in the area began to melt and retreat. As it did so, seawater flooded into the valley to create the fjord. Today, the fjord continues to receive a large input of fresh water from glacier melt. Throughout much of its length, the fjord is stratified into a lower layer of salt water, which moves into the fjord during flood tide, and an upper layer of fresher water that flows outward to the sea on the ebb tide.