Answer : Over a period of time mountain ranges will undergo soil erosion or surface run off due to rains on mountains; the soil over the mountain will be washed away along with the wind or water and get deposited in the nearby valleys.
This process continues and the place from where the soil is eroded gets deposited with another layer of soil from top of the mountain from where the soil erosion occurs.
This cycle continues and the erosion and deposition process also keeps going on.
