During the last ice age period, the water level in the lake dropped, and it became a salt lake. This made the soil alkaline, which helped to preserve the remains left behind in the Walls of China. Although the lake completely dried up several thousand years ago, ground vegetation remained on the Walls. This which helped to stabilise them and preserve them. With the arrival of European settlers in the area in the 1880s, introduced species, mostly rabbits, sheep and goats have destroyed the vegetation cover. This has led to increased erosion of the dunes. However, this erosion has led to the uncovering of many human and animal remains. Wind has moved sand and soil eastwards from the Walls, forming a mobile sand dune which moves further east every year.