The physical geography of the Basque Country is very diverse despite the small size of the region. The territory hosts a blend of green and brown to yellowish tones, featuring hilly terrain altogether. The Basque Country spreads from the rough coastal landscape to the semi-desert of the Bardenas on the south-eastern fringes of Navarre. The main natural limits of the Basque Country are the Cantabrian Sea (south Bay of Biscay) in the north and in the west side, and in the south side, the Ebro river. The highest point in the Basque Country is the Hiru Erregeen Mahaia peak (2438 m high). Coast The coast of the Basque Country has three sides. The coast of Biscay is rough but in general there are big beaches. There are also at least two big estuaries, that of Ibaizabal (known also as the Ría de Bilbao) and Urdaibai. The coast of Gipuzkoa shows a more rugged terrain and in general the slopes are steeper. From the limit of Biscay to Zumaia there are smaller beaches and they are more rocky. East of Zumaia, they are larger and longer. When different rivers join the sea, small estuaries can be found (Oria), and in the others, maritime branches (Urumea, Bidasoa). The coast of Lapurdi is flatter. The beaches are longer and the slopes gentler. The rivers Bidasoa and Adour join the ocean in Lapurdi. Mountains The Basque country is mountainous. Most of the mountains are along an east-west axis, at the west of the Pyrenees. The most important rock there is limestone, but there are also mountains composed of other materials. For example, Aiako Harria is composed of granite. The highest mountain is Hiru Errege Maila. The south of Álava and Navarre is the middle east limit of Cantabrian Mountains. There are mountains like Kodes and Toloño. Between those importants mountains there are Euskal mendiak: Urbasa, Andia, Aralar, Anboto, Ordunte or Aizkorri. Rivers The Ebro is the largest river which crosses the Basque Country. It is 910 km long, and flows along the southern boundary of the Basque Country before it joins the Mediterranean sea. On its way east, the Ebro harvests the water of a string of tributaries, such as Zadorra, Baias, Ega and Aragon. Aragon's course is the longest, at 192 km. The river rises in Astun, located in Aragon; it flows down past the Esa reservoir, and crosses thereafter eastern Navarre until