The Eastern Cordillera of Colombia is a branch of the Andes. It is composed of rocks ranging in age from Lower Paleozoic to Recent. Lower Paleozoic phyllites are overlain by unmetamorphosed Devonian to Jurassic sediments. A geosyncline formed in late Jurassic times and continued to influence sedimentation until the close of the Cretaceous. A great thickness of marine sandstone, limestone, and shale was deposited in the geosyncline, which was affected by periodic transgressions and regressions of the sea and minor diastrophic movements. The final retreat of the sea at the end of the Cretaceous period led to the development of a nonmarine environment in which the Tertiary sediments were deposited.

The structure of the Eastern Cordillera results primarily from orogenic movements in late Miocene times, although there were some earlier periods of diastrophism. The central parts of the chain are characterized by generally simple folding and subsidiary faulting. The margins, which are more strongly deformed, are cut by important thrusts that dip toward the center of the chain. Some of them have very large displacements.

The Eastern Cordillera was strongly uplifted in Pleistocene times with the result that the base of the Tertiary is now as much as 8000 m higher in the Cordillera than in the adjoining basins.

The chain is symmetrical and comparatively simple. It may be explained in terms of primarily vertical movements of the earth's crust.

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