A fold mountain is one created by the bending of strata, a tectonic process. When continental plates collide with one another or with oceanic plates, their margins exert immense compressive forces that buckle the Earth’s crust and crumple sedimentary layers. This slow deformation results in a corrugated series of folds called synclines (concave) and anticlines (convex). Differential erosion of these rock structures can make fold mountains out of anticlines, and low points in the local relief from synclines, but can also produce anticlinal valleys and synclinal ridges. When erosion sheers off the humps of the folds, it may leave sharp-edged scarps that resemble fault-block mountains. Actual faulting takes place under extreme pressure, as does metamorphosis, turning shale into slate, sandstone into quartzite, and limestone into marble. The Appalachians are old fold mountains created by the collision of two continental plates, then exposed by the erosion of thousands of feet of strata laid down above. One sees the north-south trend of these ancient folds when traversing the ridge-and-valley region of Pennsylvania. The Rockies and Andes arose from complex interactions between oceanic and continental plates.