Geologically, a fjord or fiord (English pronunciation: i/'fj??rd/ or i/fi'??rd/) is a long, narrow inlet with steep sides or cliffs, created by glacial erosion.[clarification needed] There are many fjords on the coasts of Alaska, British Columbia, Chile, Greenland, Iceland, the Kerguelen Islands, New Zealand, Norway, Labrador, Nunavut, Newfoundland, Scotland, and Washington state. Norways coastline is estimated at 29,000 kilometres (18,000 mi) with fjords, but only 2,500 kilometres (1,600 mi) when fjords are excluded. A fjord is formed when a glacier cuts a U-shaped valley by ice segregation and abrasion of the surrounding bedrock. Glacial melting is accompanied by the rebounding of Earths crust as the ice load and eroded sediment is removed (also called isostasy or glacial rebound). In some cases this rebound is faster than sea level rise. Most fjords are deeper than the adjacent sea; Sognefjord, Norway, reaches as much as 1,300 m (4,265 ft) below sea level. Fjords generally have a sill or shoal (bedrock) at their mouth caused by the previous glaciers reduced erosion rate and terminal moraine. In many cases this sill causes extreme currents and large saltwater rapids (see skookumchuck). Saltstraumen in Norway is often described as the worlds strongest tidal current. These characteristics distinguish fjords from rias (e.g. the Bay of Kotor), which are drowned valleys flooded by the rising sea. Drammensfjorden is cut almost in two by the Svelvik "ridge", a sandy moraine that during the ice cover was under sea level but after the post-glacial rebound reaches 60 meters above the fjord. During the winter season there is usually little inflow of freshwater. Surface water and deeper water (down to 100 meters or more) are mixed during winter because of the steady cooling of the surface and wind. In the deep fjords there is still fresh water from the summer with less density than the saltier water along the coast. Offshore wind, common in the fjord areas during winter, sets up a current on the surface from the inner to the outer parts. This current on the surface in turn pulls dense salt water from the coast across the fjord threshold and into the deepest parts of the fjord.