The Tajo River basin in the province of Madrid is a basin of limestone, gypsum, marl and sandstone sediments. The limestone is hard and has resisted erosion over time, forming large plateaus and agricultural wastelands known as alcarrias. On the other hand, the gypsum and marl are much softer materials and environmental precipitation has created a curious landscape of slopes, cliffs and test hills that precipitate dissolved salts to the bottom of the plains, determining the reliefs, textures and highly specialised plant species, with numerous peninsular endemic species.