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 and made part of the mainland by sedimentation of the intervening
graben.