A fjord is an inlet of a stream valley which has been modified by glacial erosion and then partially filled by the sea. The sides are extremely steep and often contain hanging valleys. These glacial troughs are U-shaped and the water is unusually deep. In the fjord, a narrow strip of land called a strandflat is usually the only suitable location for towns and ports.
The fjord usually contains a threshold or sill, often near the terminus or mouth, which is much shallower than the headward portion of the trough. If the sea level were lower than this sill, the fjord would become an elongated basin containing a lake. This high point may be caused by deposition or by more effective erosion of the head of the fjord where thicker ice existed.