A rift valley is a deep, elongated depression in the Earth's crust that is formed by the separation of two or more tectonic plates. Rift valleys are typically found in areas where the Earth's crust is thin and weak, such as along plate boundaries. As the plates move apart, the crust stretches and thins, forming a deep valley. Rift valleys can be several kilometers deep and hundreds of kilometers long. Some of the most famous rift valleys include the Great Rift Valley in Africa and the Rio Grande Rift Valley in the United States.