As much as 12,000 m of nonmarine Cenozoic deposit is preserved in the Upper Magdalena valley between the Central and Eastern Cordilleras in southwestern Colombia. Near the end of the Cretaceous period early Andean deformation uplifted the Central Cordillera. Concomitantly the sea began to retreat from the East Andean geosyncline whose axis lay along the site of the Eastern Cordillera. After withdrawal of the sea the latest Cretaceous and Paleocene Guaduas formation, having an average thickness of 1,000 m, accumulated on a poorly drained lowland stretching eastward to the Llanos. It consists of drab, mottled, and reddish-brown mudstone, and dark-gray sandstone with abundant dark grains of Cretaceous chert and mudstone. Carbonaceous deposits are more common than in any other Cenozoic formation. During the Cenozoic era a succession of disturbances generated four major alternations of coarse- and fine-grained deposits. The Central Cordilleran provenance persisted, but detritus also was derived increasingly from local sources within the basin and on the site of the Eastern Cordillera, and the Cretaceous sedimentary mantle progressively was stripped from basement blocks.