The seabeds of Capo San Marco – Sciacca are characterized by a coastline with a wide and gentle sloping plateau which consists of calcarenitic sediments overlaid on the Pliocene and pre-Pliocene substrate. Toward Capo San Marco, the coast becomes rugged and the clayey scarp descends beyond -20 meters. About 700 meters from the shore, the “Secca di Capo S. Marco” extends with a minimum depth of 9 meters; it is followed by another rocky ridge at 1,500 meters down to -28 meters which is separated by a narrow muddy canyon.
The promontory slopes gently towards the east, with cliffs enclosing a shallow area with lagoon-like characteristics which include “barrier reefs” of Posidonia and Cymodocea, fostering significant biodiversity. The area holds great importance in terms of landscape and environmental biology due to the extensive Posidonia meadows acting as nurseries for fish species. Moreover, this meadows create some barrier reefs in shallow areas, resulting in a shoal with lagoon characteristics.