What are the pre-Pyrenees?

The pre-Pyrenees are strips of relatively recent folded mountain terrain located to the north and south of the Pyrenees' central axial massifs. These formations originated when the Iberian and European tectonic plates collided, lifting secondary sediments into parallel chains that now serve as great spurs of the main mountain range.12

The structure of these ridges varies by side:

• Northern side: An outer strip of folds forms the Petites Pyrénées, which are cut by river channels.1
• Southern side: The pre-Pyrenean series is more highly developed and thicker than its northern counterpart. It includes the Interior Ridges (such as Mount Perdido), a middle depression containing various marine and continental deposits, and a fringe of Exterior Ridges that form the northern rim of the Ebro depression.1

While the central axial zone of the Pyrenees is composed of older, rigid rocks like granite and schist, the pre-Pyrenean zones consist of more recent, plastic layers that folded under tectonic pressure without breaking.2