 Just behind it is a zone of crustal thinning caused by extensional forces; i.e., the subduction and the rotation cause a wave of compression with a peak under the Anti-Apennines and a valley in the Tyrrhenian Sea.The result is a karst-graben or half-graben landform with the Volscian Mountains as karst and the coastal chain of Pontine marshes, South Pontine marsh, Terracina Basin, Gaeta Basin and Volturno Basin as graben.This landform began to appear in the Messinian stage of the Miocene, about 7.2 to 5.3 million years ago.