A valley is a type of landform. A valley is a lower part in the land, between two higher parts which might be hills or mountains. Valleys often start as a downward fold between two upward folds in the surface of the Earth, and sometimes as a rift valley. A valley is made deeper by a stream of water or a river as it flows from the high land to the lower land, and into a lake or sea. Some valleys are made by glaciers which are slow-moving rivers of ice. When water or ice make a valley deeper or wider, this is called erosion. Wind can also make valleys larger by erosion.