Other types of landforms are shaped by erosion and deposition. For instance, canyons are deep, narrow valleys with steep sides, often carved by rivers. Deltas are landforms created by deposition of sediment that is carried by a river as the flow leaves its mouth and enters slower-moving or standing water. Coastal features like beaches, cliffs, and islands are also considered landforms, shaped by the interaction of land and sea. Even features beneath the ocean, such as ocean trenches and mid-ocean ridges, are recognized as significant landforms. The study of these features and how they are formed is known as geomorphology.