Wind creates special shapes in nature. These include the elongated dunes of the sandy deserts, which are always reshaped by the wind and the treetops, which adapt their appearance to the constantly blowing wind. With its constant force the wind also erodes the surface of the rocks and hollows them out gradually. This is known as wind erosion. This gives rise to rocky arches, troughs, and spires. Wind erosion also takes place in the unprotected fields, where the fertile soil cover is blown away. In this way, the wind contributes to the spread of deserts in the dry regions of the Earth.