Sierra Madrona, lies outside of Andalusia. Within the Sierra Morena, the gorge of Despeñaperros forms a natural frontier between Castile and Andalusia. The Baetic Cordillera consists of the parallel mountain ranges of the Cordillera Penibética near the Mediterranean coast and the Cordillera Subbética inland, separated by the Surco Intrabético. The Cordillera Subbética is quite discontinuous, offering many passes that facilitate transportation, but the Penibético forms a strong barrier between the Mediterranean coast and the interior. The Sierra Nevada, part of the Cordillera Penibética in the Province of Granada, has the highest peaks in Iberia: El Mulhacén at and El Veleta at . Lower Andalusia, the Baetic Depression, the basin of the Guadalquivir, lies between these two mountainous areas. It is a nearly flat territory, open to the Gulf of Cádiz in the southeast. Throughout history, this has been the most populous part of Andalusia. Hydrography Andalusia has rivers that flow into both the Atlantic and the Mediterranean. Flowing to the Atlantic are the Guadiana, Odiel-Tinto, Guadalquivir, Guadalete, and Barbate. Flowing to the Mediterranean are the Guadiaro, Guadalhorce, Guadalmedina, Guadalfeo, Andarax (also known as the Almería) and Almanzora. Of these, the Guadalquivir is the longest in Andalusia and fifth longest on the Iberian peninsula, at . The rivers of the Atlantic basin are characteristically long, run through mostly flat terrain, and have broad river valleys. As a result, at their mouths are estuaries and wetlands, such as the marshes of Doñana in the delta of the Guadalquivir, and wetlands of the Odiel. In contrast, the rivers of the Mediterranean Basin are shorter, more seasonal, and make a precipitous descent from the mountains of the Baetic Cordillera. Their estuaries are small, and their valleys are less suitable for agriculture. Also, being in the rain shadow of the Baetic Cordillera means that they receive a lesser volume of water. The following hydrographic basins can be distinguished in Andalusia. On the Atlantic side are the Guadalquivir basin; the Andalusian Atlantic Basin with the sub-basins Guadalete-Barbate and Tinto-Odiel; and the Guadiana basin. On the Mediterranean side is the Andalusian Mediterranean Basin and the upper portion of the basin of the Segura. Soils The soils of Andalusia can be divided into three large areas: the Sierra Morena, Cordillera Subbética, and the Baetic Depression and the Surco Intrabético. The Sierra Morena, due to its morphology and the acidic content of its rocks, developed principally relatively poor, shallow