The Rocas Sedimentarias de Monte represent the record of a marine sedimentological event with high subaerial volcanic influence in the Upper Magdalena Valley. Recent detailed studies of this unit, which crops out at the Monte area, close to the town of Natagaima (Tolima), have allowed to precise the lithological conformation, the thickness and the relations with the adjacent units; these characteristics are described in this paper. The sequence of the Rocas Sedimentarias de Monte Frio, of 423,5 m thick, is subdivided into 6 segments to detail its conformation; in general, the succession consists of intercalationsof micritic limestones, very fine grained sandstones, siltstones, mudstones, chert, tuffs and occasionally intraformational conglomerates; the predominant color of the rocks is black and dark gray. The lower contact is covered in the area, and the upper part is concordant and transitional with the volcanic red beds of the Saldana Formation. The depositional environment was variable from shallow marine to subaerial with a high volcanic and tectonic influence which caused instability in the depositional basin. The presence of liasic ostracodes and conchostraca in the Rocas Sedimentarias de Monte Frio locates its stratigraphy over the Payande Formation and the Chicala Member; although it is not discarded that in this sequence can be included some parts of both of these units. It is recomended to consider the Chicala Member and the Rocas Sedimentarias de Monte Frio within the Payande Formation to make easier the differentiation from the Saldana Formation.