Valencia occupies a narrow coastal strip extending from the Ebro delta to the mouth of the Rio Segura - though the province of Alicnte, south of Cabo de la Nao, belongs geographically to Murcia. Here the treeless reddish-grey limestone and sandstone plateau of the Meseta approach the sea and terminate in steep rugged coastal cliffs slashed by narrow river gorges. The rivers flowing down from the interior, like the Guadalaviar and the Júcar, which surge down in great spates after thunder showers or when the snow melts, depositing fertile alluvial soil along the coast, provide water for the irrigation of the thirsty lands lying in the rain shadow of the hills. The irrigation system, first constructed by the Romans and later developed by the Moors, makes Valencia the most fertile part of Spain. An ancient code of regulations provides for the equitable distribution of the precious water, which is carried far and wide throughout the region in a a network of countless canals and smaller channels and stored up during the winter in reservoirs against the summer drought.