Block diagrams showing changes due to valley glaciation. (A) Unglaciated mature topography. (B) A system of valley glaciers occupies the former stream valleys and causes deep erosion. Frost action causes weathering of unglaciated slopes and provides a source of debris for lateral moraines. (C) Climatic change causes melting and disappearance of glaciers, leaving behind tell-tale topography which includes hanging valleys. U-shaped valleys, lakes, end moraines, and rugged peaks.