This document defines and describes various landforms including oceans, islands, valleys, mountains, caves, glaciers, streams, rivers, plains, and isthmus. It notes that oceans include the Pacific, Indian, and Southern Oceans. Islands are land surrounded by water, valleys are areas between hills, and mountains may form from volcanoes. Caves are formed from limestone and rock, and some glaciers contain caves. Streams flow like paths of water and rivers lead to the sea. Plains are very flat land, while an isthmus connects two land areas surrounded by water.