The diapirs of the western Pyrenees and their foreland have cores mainly of salt and evaporites of Triassic (Keuper) age. Their shapes and tectonic positions differ. They are surrounded or overlain mainly by Cretaceous and Tertiary sediments. Groups of diapirs demonstrate distinct alignments. The distribution of the diapirs is believed to be controlled by variation in the thickness of the Upper and Lower Cretaceous sediments. These sediments, which reach a maximum thickness of at least 8,000 m., exerted the necessary pressure to start the movement of the saliferous beds towards the flanks of the trough. Shifting of the trough axis in Upper Cretaceous time separated the salt accumulation into two distinct welts. Diapirism started in early Cretaceous time and must have reac ed its maximum activity during the late Cretaceous because most of the diapirs had reached the surface prior to late Tertiary time.

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