Look at an old stone wall, a worn gravestone or a crumbling cliff. Hard rock does not stay hard forever. Rain, ice, air and living things slowly turn it into smaller pieces. This is weathering, and it is the first step in the story of every landscape: it makes the loose material that rivers, glaciers and wind can later move.

The key idea of this lesson is one sentence: weathering breaks rock down where it sits; erosion carries the pieces away. We will meet erosion in a few lessons. For now, remember that weathering does not need any movement at all.