Nave dos Cordeiros is a polje-type karst structure, a symmetrical elongated depression about 5 km long, aligned with the Nave do Barão along an East-West fracture and separated by a small elevation, called portela. This polje developed in Lower Jurassic limestones and dolomites, with its bottom being covered by colluvium and terra-rossa, which allow the formation of temporary lagoons during the rainy season. In the villages of Nave dos Cordeiros and Charneca da Nave it is also possible to find residual reliefs formed by oolitic limestone. The polje is crossed by the Alte river, which originates in Fonte Grada and flows into Vala Grande. In very rainy winters, the bottom of the polje can also be covered with water from springs at the bottom or from the slopes, due to the rise in the water table.