n.
or fiord
Long, narrow arm of the sea, often extending well inland, that results from marine inundation of a glaciated valley.
Many fjords are remarkably deep; it is assumed that the huge glaciers that formed in these valleys were so heavy that they eroded the bottoms of the valleys far below sea level. After the glaciers melted, the waters of the sea invaded the valleys.
Bradshaw Sound, Fiordland, west coast of South Island, New Zealand
By courtesy of the New Zealand Geological Survey; photograph, T. Ulyatt