Peneplain is an area that was once hilly or mountainous, but which has been almost flattened by erosion. Peneplains form in humid climates. Streams wear away rock and soil from the walls and floors of narrow, V-shaped valleys. Erosion continues for millions of years—until the streams have cut to the lowest possible level. All that remains is a gently rolling plain. In some cases, low, widely spaced peaks called accordant summits rise above this surface.
Movements in Earth’s crust may raise the level of a peneplain, or a peneplain may rise due to fractures and folds in the crust. Erosion will then start again.
