The geographic feature that is a deep valley cut into the earth's crust is called a **rift valley**. Rift valleys form when tectonic plates diverge or separate, creating an elongated depression. This geological phenomenon is often characterized by steep walls and can be accompanied by volcanic activity. One of the most notable examples of a rift valley is the *Great Rift Valley* in East Africa, which stretches from Syria to Mozambique and plays a significant role in the region's geology and ecology. These areas can also be rich in biodiversity and unique habitats.