Rift valleys can be very deep, and both such valleys known to exist in continental areas on Earth, the East African Rift and the Siberian Baikal Rift, have large, deep lakes in them. As the walls of a rift valley are pulled apart, the floor of the valley tends to sink. If this subsidence happens faster than erosion from the walls can fill it in with sediment, the floor of the valley continues to fall relative to the surrounding terrain.