The term 'gradually wore down' can be summarized by the word 'eroded', which has 6 letters. This term is often used to describe the process of a material or surface being worn away over time due to natural forces such as wind, water, or ice. This process can also be caused by human activities such as farming, construction, and mining. Erosion is an important part of the Earth's natural cycle and can have both positive and negative effects on the environment.