Portugal is an extension of the mountainous formations of the Iberian Peninsula. In its terrain two types of relief are distinguished: the South and North of the river Tagus. In the north the relief is more rugged, slopes are steep and cut by deep valleys. The weather sculptured higher grounds, giving them a distinct look of the Spanish landscape. Between the rivers Minho and Douro lies a mountanious chain, that ramifies itself upto the the coastline and leaving a narrow marginal band behind. Between the Douro and the Tagus are raised the highest peaks: Serra do Marão and Serra da Estrela (6 539 feet/ 1 993 meters). In the South of the river Tagus, the land is more uniform, marshy and of scarce relief. The predominant vegetation, in this region, is limited to plants and shrubs (charnecas) or mediterranean trees (cork-trees, scarlet-oaks, fig-trees and olive-trees). The hydrographic network is rich, gaining discharge and importance in rivers that begin in Spain, as the rivers Minho, douro, Tagus and Guadiana that flow into the Atlantic Ocean. The estuaries of the Douro and the Tagus constitute important anchorages, having bestowed on the Portuguese nation an important natural platform for discovery and overseas commerce.