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Geology

The Pyrenees are older than the Alps: their sediments were first deposited in coastal basins during the Paleozoic and Mesozoic eras. During Ediacaran to Ordovician times, Pyrenees were located at the Northwest margin of Gondwana, where they formed a lateral continuity of neighbouring areas, such as the Montagne Noire and the Mouthoumet massifs and Southwestern territory of Sardinia. Between 100 and 150 million years ago, during the Early Cretaceous Period, the Bay of Biscay fanned out, pushing present-day Spain against France and applying intense compressional pressure to large layers of sedimentary rock. The intense pressure and uplifting of the Earth's crust first affected the eastern part and moved progressively to the entire chain, culminating in the Eocene Epoch.

The eastern part of the Pyrenees consists largely of granite and gneissose rocks, while in the western part the granite peaks are flanked by layers of limestone. The massive and unworn character of the chain comes from its abundance of granite, which is particularly resistant to erosion, as well as weak glacial development.

The upper parts of the Pyrenees contain low-relief surfaces forming a peneplain. This peneplain originated no earlier than in Late Miocene times. Presumably it formed at height as extensive sedimentation raised the local base level considerably.