At the end of the Cretaceous, the tectonic regime changed to a compressive phase, due to the movement of the Caribbean Plate. The Early Paleocene deposits of the Hato Nuevo and Manantial Formations show a more calcareous character in the north, while the Cesar Sub-basin contained more siliclastic sedimentation, represented in the Barco Formation, consisting of more lithic fragments than the equivalent of the Llanos Basin. Compression continued during the Paleocene, with uplifted areas to the northwest and southeast and volcanism in the proto-Caribbean. The global climate was very hot in this period and in the restricted basin between the two forming mountain ranges, a unique ecosystem developed; the first Neotropic forest. In this hot and humid environment, the largest species of reptiles since the extinction of the dinosaurs evolved, of which Titanoboa was the main predator. It has been estimated on the basis of the fossil flora, pollen and large reptiles that the mean annual temperature was between and yearly precipitation ranging from per year. Provenance analysis of the sediments of the Los Cuervos and Cerrejón Formations show a predominant west to east paleocurrent, followed by a more southeastern flow. A secondary source of sediments was the growing Serranía del Perijá.