What is it called when softer rocks wear away and leave harder rocks behind?

   Differential weathering

Explanation:  Depending on the composition of the wall material, depend the wall resistance to time changes and all the factors that lead to devastation or erosion. Thus the composition of the materials of the rock, determines which speed will lead to the disintegration of the wall. Thus, the stone, whose material is more resistant to weather changes, will remain behind, while the stone with a softer composition will devastate and decompose to smaller pieces and wear away. This leads to a differential weathering of the walls and  to the formation of an uneven surface.

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answer; environmental science;