Monte Circeo or Cape Circeo is a mountain remaining as a promontory that marks the southwestern limit of the former Pontine Marshes. Although a headland, it was not formed by coastal erosion–as headlands are usually formed–but is a remnant of the orogenic processes that created the Apennines. The entire coast of Lazio, on which the mountain and the marsh are located, was a chain of barrier islands that was formed on a horst, made part of the mainland by sedimentation of the intervening graben.