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The Pyrenees are older than the Alps: their sediments were first deposited in coastal basins during the Paleozoic and Mesozoic eras. In the Lower Cretaceous period, the Gulf of Gascony (Bay of Biscay ) fanned out, pushing Spain against France and putting large layers of sediment in a vice grip. The intense pressure and uplifting of the Earth's crust first affected the eastern part and stretched 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.