Imagine a flat land surface. Assume the processes acting to erode the land are uniform throughout the area. Note that the resistance of these materials to erosion varies from place to place.
As a result, the elevation of the areas where less resistant material occurs will be reduced more quickly than the elevation of the areas where the more resistant materials occur. This effect is called 'differential erosion'.
The result of differential erosion is that as time passes, the places where less resistant materials occur will become lower than those where more resistant materials occur.
Thus, as the land is being worn down, some parts will be worn away less rapidly and may remain as ridges, hills or mountains. They are erosional 'remnants' of the earlier landform.
Other parts will be worn away more rapidly and will become valleys or lowlands.
David J. Leveson