From Valanginian until Campanian or Maestrichtian time, the Andean belt in central Peru consisted essentially of two parallel northwest-trending troughs separated by a belt of deformed and metamorphosed pre-Mesozoic rocks, the so-called Maranon geanticline. Until the late Aptian, clastic sediments accumulated under mainly nonmarine conditions; in early Albian time a major marine transgression began in the western trough, inundating the geanticlinal region by the mid-Albian and advancing into the eastern trough during the Cenomanian, Turonian, and Coniacian. Limestones, dolomites, and shales were deposited. In the western trough, volcanism beginning during Albian time is evidenced by thick sequences of interbedded pyroclastic rocks and shales. Emergence occurred in the Santonian and red beds were deposited to the east (and locally to the west) of the present continental divide. By Maestrichtian time, marine conditions prevailed only in north and northwest Peru. Data on the stratigraphic and facies relations, faunas, and correlation of the various Cretaceous formations are presented.