Landform

In addition to producing its own distinctive land-forms, deformation of Earth's crust is influential in controlling what landforms result from differential weathering and erosion. During major mountain building episodes huge volumes of rock are compressed, folded into complex three-dimensional forms, and sometimes metamorphosed into different kinds of rocks. Later, when these folded layers are exposed to the agents of weathering and erosion, the more resistant units become ridges which outline the folds and deformation. Often resistant units offer better protection at the bottom of the fold than they do at the top, resulting in landforms with higher elevations over what were the troughs in the folds, and lower elevations over what were the crests. Much of the valley and ridge areas of Pennsylvania and adjacent states have this kind of landform.