The Andes Mountains divide just north of Colombia's southern border with Ecuador into three separate chains, or cordilleras, known as the Cordillera Occidental (western), the Cordillera Central, and the Cordillera Oriental (eastern). The western and central cordilleras run roughly parallel with the Pacific coast, extending northward as far as the Caribbean coastal lowlands. They are alike in geological structure, both being composed of massive crystalline rocks. The Cordillera Central is the highest range of the Colombian Andes, with several volcanic cones whose snow-covered peaks rise to about 5,500 m (18,000 ft), notably Huila (5,750 m/18,865 ft). The third chain, the Cordillera Oriental, runs northeastward, bifurcating into an eastern branch, the Sierra de los Andes, which slopes down to Venezuela, and a second branch, the Sierra de Perijá, which continues northward to terminate on the border between Venezuela and Colombia just south of the Guajira Peninsula. This range is composed of folded stratified rocks over a crystalline core. On the margin of the Caribbean stands the Sierra Nevada de Santa Marta, an isolated block of mountains composed of a triangular massif of granite, whose highest elevation is Pico Cristóbal Colón (5,775 m/18,947 ft), the tallest peak in Colombia. West of the Cordillera Occidental but not geologically a part of the Andean chain is the low Serranía de Baudó, which skirts the Pacific and extends into the Isthmus of Panama.
Separating the three principal Andean ranges are Colombia's two major rivers, the Cauca (1,014 km/630 mi), which flows northward between the western and central cordilleras, and the Magdalena (1,553 km/965 mi), which divides the central and eastern cordilleras. After emerging from the mountains, the two rivers become one and descend through marshy lowlands to the Caribbean. The area south and east of the Andean ranges is largely composed of river plains divided among the effluents of the Orinoco and Amazon rivers. Open plains immediately adjoin the mountains, but as the distance from the cordillera increases, the plains give way to largely uninhabited and unexplored jungle. The Pacific coastal area is also characterized by jungle vegetation. Principal rivers on the Pacific coast include the Baudó, San Juan, and Patía.