Two billion years ago, the Continent of Columbia included what are now the Amazonian Platform and the West African Craton in addition to other land fragments. Three hundred million years later, a thick layer of sandstone was deposited across what would become the northern section of the Amazon Platform, likely eroding from an adjacent mountain range; today, this 1.7 billion year old Precambrian sandstone formation is known as the Roraima Plateau.
When Pangea formed, late in the Paleozoic Era, Columbia was incorporated within its mass. Then, as Pangea rifted apart during the Triassic (about 220 MYA), Columbia became part of Gondwana (the massive southern Continent); when the Atlantic Ocean began to open during the Jurassic (150 MYA), the West African Craton was split from the Amazonian Platform. Breaking from Gondwana late in the Cretaceous (about 70 MYA), South America drifted westward and, throughout the Cenozoic Era, tectonic forces have continued to shape the Continent. The Amazon Graben produced the valley of the Amazon River, splitting the Amazon Platform into northern and southern sections (the Guiana and Brazilian Shields, respectively). The Guiana Shield now covers northeastern South America, from eastern Venezuela and southeastern Colombia to the Atlantic Ocean; its northwestern edge is bordered by the Orinoco River Valley while the Amazon River Valley forms its southern border. The numerous tributaries of these rivers (and of other coastal streams) have eroded the Guiana Shield, leaving highlands sculpted from the Roraima Plateau; among these are tall mesas known as tepuis, most numerous across western portions of the Shield. Cenozoic sediments, eroded from the Guiana Shield, now fill upper segments of the Orinoco and Amazon Basins, as do volcanic desposits from the Andes to the west.
Today, the Guiana Shield is covered by one of the most extensive and least disturbed tropical rain forests on Earth, home to a spectacular diversity of plant and animal life (including more than 20,000 species of vascular plants). Its magnificent topography, the product of torrential rains acting on ancient Precambrian rock, is renowned for its numerous waterfalls; among these is Angel Falls in Venezuela, the tallest cascade (3212 feet) on or planet.
