The Monti Aurunci mainly consist of
friable limestone, which becomes harder toward
Gaeta. The degree of faulting and cracking is so high that the mountains retain no rainfall; it sinks in to emerge as springs (and used as wells) on the lower flanks. The stream beds are dry except for vernal pools. Most generally, the western-central coastal region of Italy is the front of a
subduction zone where the
African Plate moving locally from southwest to northeast is carried under the
European Plate. There is some counterclockwise rotation of Italy; hence the faults in the
Tyrrhenian Sea slip both parallel to the shoreline and perpendicular to it. The surface rock in the
Anti-Apennines was deposited on the floor of
Tethys Sea during the
Jurassic and
Cretaceous of the
Mesozoic. This lighter calcareous rock rides over the front of the subduction zone, uplifted by compressional and
isostatic forces. 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. It went on to mature in the
Pliocene. Also in this time volcanic activity associated with the faults and the weakening of the crust over the subduction created the volcanic zones of
Latium and
Campania, which intruded into the karst-graben, mainly on the karst side. In the
Pleistocene the basins slowly filled with sediment from the weak run-off of the mountains, accelerating with the deforestation of modern times. ==Ecology==