What is a rift valley?

A rift valley in geology is a valley created by the formation of a rift. or Valley formed by the subsidence of a block of the Earth's crust between two or more parallel faults. Rift valleys are steep-sided and form where the crust is being pulled apart, as at mid-ocean ridges, or in the Great Rift Valley of East Africa. In cross-section they can appear like a widened gorge with steep sides and a wide floor.