The Cagayan Valley basin is a north-south trending interarc basin in which 10,000 meters of marine, transitional marine, and fluvial sediments have been deposited since the Oligocene. The Plio-Pleistocene sediments have been divided into two lithostratigraphic units, the Plio-Pleistocene Ilagan Formation and the Pleistocene Awidon Mesa Formation. The Ilagan Formation is divided into two members, a Lower Member formed by 310 m of interbedded sandstone and mudstone, and an Upper Member formed by 500 m of pebble conglomerates, sandstones, siltstones, tuffs, and claystones. The Awidon Mesa Formation conformably overlies the Ilagan Formation and is composed of 400 m of pebble to boulder conglomerates, tuff-breccias, tuffs, sandstones, and claystones. The sediments which were derived from volcanic and plutonic igneous rocks in the adjacent volcanic arcs are predominantly composed of volcaniclastic detritus which has been diagenetically altered by dissolution and cementation processes. Five major lithofacies are recognized in the Plio-Pleistocene sequence. The facies and respective depositional environments are: (1) the interbedded fine grained sandstone and mudstone facies: delta front distal bar and distributary mouth bar; (2) the lenticular cross bedded, medium grained sandstone and siltstone facies: delta plain distributary channel, levee, and flood basin; (3) the polymictic conglomerate, trough cross bedded sandstone and claystone facies: low energy fluvial channel and floodplain; (4) the clast supported polymictic conglomerate and sandstone facies: high energy channel bar and gravel sheet; and (5) the massive matrix supported pebble to boulder conglomerate, tuff-breccia, and tuff facies; lahar, pyroclast flow and fall. The facies record a regression of the Pliocene Sea as the region was uplifted and the basin filled with detritus from the surrounding volcanic arcs. A lobate high constructive delta complex prograded to the north as it received sediment from a large north flowing meandering stream system. In the Pleistocene geanticlinal uplift of the Cordillera Central volcanic arc resulted in the formation of an alluvial fan complex along the western side of the valley. Concomitant plinian and pelean volcanic eruptions in the Cordillera produced pyroclast flows which spread across the fan and into the valley. In the middle Pleistocene, the Cagayan Valley alluvial plain supported a tree savannah ecosystem and large grazing mammals which migrated from Asia via a land-bridge. Paleolithic stone tools found in surface associations with fossilized remains of the middle Pleistocene fauna suggest that hominids also migrated to the Philippines in the Pleistocene. Thus far, no skeletal evidence of hominids has been found in the sediments which were folded in the middle to late Pleistocene due to gravity sliding of unstable uplifted sediments.