The Pyrenees are a range of mountains in southwest Europe that form a natural border between France and Spain. They separate the Iberian Peninsula from France, and extend for about 430km (267 mi) from the Bay of Biscay on the Atlantic Ocean to Cap de Creus on the Mediterranean Sea.
For the most part the main crest forms the Franco-Spanish frontier, with the principality of Andorra sandwiched between them. The Pyrenees are older than the Alps: their sediments were first deposited in coastal basins during the Paleozoic and Mesozoic eras. The amount of the precipitation, including rain and snow, is much greater in the western than in the eastern Pyrenees, which leads to a marked contrast between these sections of the chain in more than one respect. A still more marked effect of the preponderance of rainfall in the western half of the chain is seen in the aspect of the vegetation. The lower mountains in the extreme west are very well wooded, but the extent of forest declines eastwards, and the eastern Pyrenees are peculiarly wild and barren.