Why are some rocks folded into waves and curves?

Folding usually happens in regions where tectonic plates collide and rocks are squeezed over long periods. At depth, higher temperature and pressure make rock deform more ductilely, so layered strata can buckle rather than fracture. The resulting shapes include anticlines, which arch upward, and synclines, which bend downward. Many mountain belts, including the Alps and Himalayas, contain large folded rock layers formed during continental collision. Once uplift and erosion expose them, these folds become visible at the surface.