Towards the south are higher mountains, the tops of which form the watershed between the drainage basins of the Rivers Ebro, Duero and the rivers that flow into the Bay of Biscay. These peaks generally exceed from the Pass of San Glorio in the west to the Pass of Los Tornos in the eastern part: Peña Labra, Castro Valnera and the mountain passes of Sejos, El Escudo and La Sía. The great limestone masses of Picos de Europa also stand out in the southwest of the region: most of their summits exceed , and their topography is shaped by the former presence of glaciers.