Anticline and Syncline, in geology, terms used to describe folded rock layers where the relative ages of the layers are known. Specifically, anticlines are folds in which the oldest rocks are in the core, or center of the fold, while synclines have the youngest rocks in their cores. In most cases, anticlines are arch-shaped while synclines are trough-shaped. In unusual cases where older rock layers lie on top of younger layers, anticlines are trough-shaped and synclines are arch-shaped. If the relative ages of the rock layers are unknown, then the terms antiform and synform describe arch and trough shapes respectively. The size of folds, measured from the crest of one arch to the crest of the next arch, can range from less than a meter (about 3 ft) to greater than 10 km (about 6 mi).
Anticlines and synclines form when rock layers are compressed, just as a carpet folds when its sides are pushed together along a floor. When rocks are compressed, they can either bend or fracture. When rocks bend they create folds, and when they fracture they can produce faults. Consequently, many folds are associated with faults.Anticlines and synclines are useful for a variety of reasons.