The term 'gradually wear down' can be summarized by the word 'ERODE', which has five letters. Erosion is a natural process in which soil, rocks, and other materials are worn away by wind, water, or other natural forces. Over time, these forces can cause gradual changes to the landscape, such as the formation of valleys, canyons, and other features. In addition, erosion can also lead to the destruction of man-made structures, such as buildings and roads.