The Cordillera Central is the highest and most formidable of the Andean ranges in Colombia, with several peaks rising above the snow line. Pico Cristobal Colon, in the extreme north, is the highest peak at 5,776 meters and among the number of active volcanoes the highest is Tolima, at 5,215 meters; it last erupted in 1829. In the extreme north, in the Sierra Nevada de Santa Marta, the Cordillera Central rise again above the snow line with the strong relief characteristic of the high Andes. This range, which is approximately 40 miles wide, has many small, populated valleys at different altitudes; taken as a whole, however, the range is only sparsely inhabited.
The Cordillera Occidental is much lower and narrower. It highest peak is only about 10,000 feet. Even in its high southern part, it has convenient passes. The range decends gently into the Caribbean coastal plain, where it becomes partly sumerged under thick beds of sediment deposited by the rivers draining the Colombian Andes. A deep and mostly narrow valley, drained by the river Cauca, runs between the Occidental and Central ranges. Like the Cordillera Central, the Occidental has only small valleys, in which most of the population lives. The Cauca and some adjoining valleys constitute one of the most densely inhabited parts of the country, with two of Colombia's four largest conurbations, Medellin and Cali.
The extensive lowlands of the east, which cover nearly two-thirds of Colombia, belong to two large drainage basins, that of the Orinoco river in the north and the Amazon river in the south, and to two natural landscape regions, the Llanos in the north and the Selvas in the south. The westernmost fringes and some outlying spurs of the Guiana Highlands and of the Andes extend into the eastern lowlands. This give the vast undulating plain most of its topographical variety and its elevated areas, which rise well above the wide floodplains of the great rivers that cross it. The rivers are a dominant feature of both the physical and human landscapes of the region. Almost the entire population of the lowlands is settled on or near river banks. Rivers, some of which are navigable up to the eastern foot of the Andes, provide the main, if not the only, access to most parts of the lowlands.