The "backbone" of Colombia is the northern part of the Andes Mountains. The mountain system is here divided into the Eastern, Central, and Western Cordilleras. The Central and Eastern Cordilleras are separated by the down-faulted basin of nonmarine Cenozoic deposition through which the Magdalena River flows in its middle and upper reaches. The valleys of the upper Cauca and upper Patia Rivers mark the approximate boundary between the Western and Central Cordilleras. The Santa Marta and Perija Mountains and the Pacific Coast Range are related to the Andean system.

The structural grain is north-northeast in the southern part of the Colombian Andes. To the north, major alignments are northeast and northwest. The Sautata arch connects the western Cordillera with the Darien Mountains of Panama. The Guaviare arch links the Andes and the Guayana shield.

The syenitic rocks and high-grade metamorphics of the Garzon-Guaviare region are the only known Precambrian west of the Guayana shield. A record of marine Paleozoic sedimentation is preserved in the Eastern Cordillera and Perija and the Macarena Mountains. Periods and epochs represented are--Late Cambrian?, Early and Middle Ordovician, Middle Devonian, Mississippian, Pennsylvanian, and Permian. There is evidence of "Caledonian" orogeny and plutonic igneous activity. No Paleozoic marine sediments have been found west of the Central Cordillera.

Interbedded marine and terrestrial sediments and volcanics of Late Triassic and Early Jurassic age crop out along the west side of the Upper Magdalena Valley, but to the northeast, only continental sediments are present. A widespread marine invasion began in latest Jurassic time and affected the Andean region and its eastern foreland during the Cretaceous. The thick marine Cretaceous deposits east of the Central Cordillera and Santa Marta Mountains are a major source of oil. The area of the Bucaramanga massif was not submerged until Aptian time. West of the Central Cordillera, Cretaceous sediments are interbedded with porphyries and diabase flows.

Orogeny began in the Maestrichtean and reached a peak in the middle or late Eocene. Basic and ultrabasic rocks were intruded in the Western Cordillera and Pacific Coast Range regions. Some of the granodiorites of the Central Cordillera may have been intruded at this time. Tertiary marine deposition was restricted to regions west and north of the Andes. A thick nonmarine Tertiary sequence elsewhere includes reservoirs containing more than 80 per cent of Colombia's proved oil reserves.

There was renewed orogeny at the close of the middle Oligocene, with extrusive igneous activity in the Central and Western Cordilleras. A thick section of marine sediments was deposited west and north of the Andes in the late Oligocene and early Miocene. Elsewhere, downwarped and downfaulted belts continued to receive continental deposits.

The most recent orogenic and volcanic activity began in early Miocene and continues today, the orogeny affecting older positive belts and the Tertiary basins of marine deposition.