The Neiva Formation (Pliocene?) marks the initiation of the youngest molasse facies in the thick, Cenozoic, nonmarine section of the southern Magdalena Valley, Colombia. This piedmont deposit is ∼200 m thick throughout most of the region. Locally, however, there was strong downwarping of the basin which permitted thicker accumulations. The sediments are very coarse grained with individual conglomeratic units >40 m thick. The conglomerate deposits consist of polymictic orthoconglomerate, with clasts of intermediate volcanic rocks the most abundant rock type. Some localities have large amounts of granitic pebbles. Volcanic arenite is the dominant sandstone type; zoned plagioclase and iron-silicate minerals are very common. Locally, subarkose is present where volcanic material is rare. The clay-mineral suite is predominantly montmorillonitic. The formation has an over-all tan color, although the mudstone beds are light olive green or light reddish brown.
Major uplift in the Central Cordillera in Pliocene time was accompanied by widespread volcanism of intermediate composition. Coarse-grained detritus was transported eastward across a regional paleoslope. The Eastern Cordillera was not a major physiographic feature in southern Colombia during deposition of the Neiva Formation. There were, however, local uplifts in the Garzón massif which supplied some granitic rocks to the basin from the east.