The São Marcos Fault- Quarteira (FSMQ) corresponds to a structure inherited from the Variscan orogenic cycle (c. 350 million years ago) and is expressed morphologically through a fracture valley, with a general NW-SE orientation, which crosses the entire aspiring Algarvensis geopark. This fault, more than 40 km long, extends from São Marcos da Serra in the direction of Paderne to the coast, in the area of Quarteira, extending to the immersed area, on the continental shelf. Two main sectors make up its layout: i) the São Marcos depression, between São Marcos da Serra and São Bartolomeu de Messines, which separates the Monchique and Caldeirão mountains; and ii) the valley between Tunes and Quarteira, corresponding to the area where the Quarteira stream settles in Meso Cenozoic formations.

Considered to be a tectonically active fault (with movement in the last 3 Ma) and probably associated with some current seismicity, it establishes the division of the Algarve into two crustal blocks with distinct tectonic behaviour: western sector, Barlavento - with less neotectonic activity; and eastern sector, Sotavento - with greater neotectonic activity.

The S. B. Messines Geosite is one of the rare places where you can observe the influence of FSMQ in an outcrop and not only in the landscape.