The Guiana Highlands are the uplands of the Guiana Shield, a Precambrian block of continental crust and one of the three cratons of the South American Plate, rising in southern Venezuela east of the Orinoco, across west-central Guyana and into Brazil's Roraima state. Their signature landforms are tepuis, sheer-sided table mountains of Precambrian quartz sandstone that stand up to 1,000 m above the surrounding forest and savanna. Mount Roraima reaches 2,810 m at the meeting point of Venezuela, Guyana and Brazil, while the highest point of the wider shield is Pico da Neblina in Brazil at 2,995 m. The highlands drain north to the Orinoco through the Caroní, east through Guyana's Mazaruni and Essequibo, and south through Brazil's Rio Branco, and they feed Angel Falls, the world's tallest uninterrupted waterfall at 979 m. Much of the Venezuelan core is protected in the 30,000 km² Canaima National Park, a UNESCO World Heritage Site since 1994. The landscape makes most sense read from the bottom up, oldest rock first.
The Imataca Complex: Archean Basement North of the Guri Fault
The foundation of the Guiana Shield is a craton, a piece of continental crust so old and stable that it has escaped the mountain-building that reshaped younger parts of the continent. It is one of three cratons in the South American Plate, and it underlies Guyana, Suriname, French Guiana, much of southern Venezuela, most of Brazil north of the Amazon and parts of eastern Colombia.
The oldest rocks in the shield belong to the Imataca Complex. They date from the Archean eon, more than 2.5 billion years ago, and consist mainly of quartz-feldspar gneiss, a banded rock recrystallised under heat and pressure deep in the crust. The Guri Fault marks the southern edge of this complex. South of it lie Early Proterozoic rocks: the volcanic rocks of the Pastora Supergroup, altered into greenstones, and the granites of the Supamo Complex. Ash-flow tuffs and further granites of the Cuchivero Group belong to the same long phase of crust-building. Together these form the crystalline basement on which everything else rests. Where it reaches the surface, it forms the rounded granite peaks of the Imataca and Parguaza complexes along the northern and south-western edges of the highlands, a sharp contrast to the cliff-walled tepuis.
The Roraima Group: Sand Laid Down 1.8 to 1.4 Billion Years Ago
On top of that basement lies the rock that makes the tepuis. The Roraima Group consists of sedimentary layers: quartz sandstones, quartzites, shales and conglomerates, all continental (non-marine) clastic deposits made of grains eroded from older land. Geologists date these sediments to roughly 1.8 to 1.4 billion years ago, in the Early to Middle Proterozoic, and the sandstone of Mount Roraima itself is placed at about 1.7 to 2 billion years old, among the oldest rocks on Earth.
Two things about this rock explain almost everything that follows. First, it is nearly pure quartz: about 98% of Roraima's grains are silica, which resists chemical weathering far better than most rocks. Second, the layers lie almost flat. Younger sheets of molten rock later forced their way between them and cooled into sills of gabbro and similar dark rock, but the sandstone layers still lie close to horizontal. A thick, flat, hard lid sitting on softer granite and gneiss is exactly the recipe for table mountains.
Gondwana's Break-up and 300 Million Years of Erosion
For most of its history the Roraima sandstone was not a set of separate mountains but a single plateau, stretching between the northern rim of the Amazon Basin and the Orinoco, and from the Atlantic coast to the Rio Negro. The region was part of the supercontinent Gondwana. According to the standard account, the plateau began to erode and break apart about 300 million years ago, and by about 70 million years ago the surviving blocks stood as isolated monadnocks, the ancestors of today's tepuis. Mesozoic layers of clay and conglomerate that once covered the Roraima summit have been stripped away by erosion and uplift over the past 180 million years.
Rain did most of the shaping. On Roraima, for instance, leaching by intense rainfall is what etched the strange summit topography, and at the mountain's southern end part of the cliff has collapsed into a giant boulder, a reminder that the walls are still being cut back. What remains are the mesas of the Gran Sabana in south-eastern Venezuela, where 115 tepuis stand within a single region with a combined summit area of about 5,000 km². Their summits range from about 1,000 m to almost 3,000 m.
Mount Roraima is the best-studied result. Its summit plateau is more than 10 km long and up to 5 km wide, covering roughly 33 to 50 km², ringed by cliffs 400 to 1,000 m high. About 85% of the mountain lies in Venezuela, 10% in Guyana and 5% in Brazil. Its highest point, 2,810 m, is on the Venezuelan southern rim; a 2,772 m point about 8 km further north is the highest in Guyana, and the marker where Brazil, Venezuela and Guyana meet stands at 2,734 m. A British expedition first reached the summit in 1884, and the relatively easy south side remains the usual route for hikers. The guide to Mount Roraima covers the climb itself.
Water Versus Quartzite: Sinkholes of Sarisariñama and Roraima's Caves
Quartzite is normally thought of as insoluble, but given hundreds of millions of years of heavy rain, water does dissolve it slowly. The tepuis therefore show features usually associated with limestone: caves, shafts, sinkholes and pinnacled surfaces, a style geologists call pseudo-karst. The same joint-widening that makes the cliffs retreat also works vertically, carving underground streams that later lose their roofs.
The most dramatic examples are on Cerro Sarisariñama, a tepui in the remote far south-west of Bolívar state with a summit area of about 547 km². Pilots first reported its sinkholes in 1954, and in 1961 the pilot Harry Gibson drew attention to the two largest. Sima Humboldt is up to 352 m wide and 314 m deep, with its own isolated forest on the floor. Other tepuis hold caves on a similar scale: the Abismo Guy Collet, 671 m deep, is the deepest quartzite cave known, and the caves inside Roraima's summit form a network more than 15 km long, including what explorers from the Oxford University Cave Club and the Venezuelan Speleological Society describe as the largest quartz cave in the world.
Rivers Off the Edge: Angel Falls, Kaieteur and the Caroní
Because the tepui summits are hard, flat and very wet, rain collects on them and leaves over the cliff edges, producing the highlands' great waterfalls. Auyán-tepui, with a summit area of about 667 km², is one of the largest tepuis, its plateau tilting from about 1,600 m in the north-west to 2,450 m in the south-east. Angel Falls drops from a cleft near its summit: 979 m in total, including an uninterrupted plunge of 807 m. Its water runs into the Churún River and then the Carrao, part of the Orinoco system. The falls are named after the American pilot Jimmie Angel, who first flew over them on 16 November 1933; their Pemon name is Kerepakupai Vená, and in 2009 the government promoted the form Kerepakupai Merú, said to mean "waterfall of the deepest place". The guide to Angel Falls has more on their discovery.
The Caroní is the main artery of the Venezuelan highlands. It runs 952 km from the Kukenan tepui to the Orinoco, the longest river entirely within Venezuela, and carries an average of 4,850 m³ per second. Like the Rio Negro, it is a blackwater river, stained dark by humic acids from decaying vegetation. About 100 km above its mouth the Guri Dam, first built between 1963 and 1969 and later raised to 162 m, holds back a reservoir of 4,250 km², and its power station was once the largest in the world by installed capacity until Itaipu overtook it.
East of Roraima, the same highland sandstone produces Guyana's Kaieteur Falls on the Potaro River, a tributary of the Essequibo. The Potaro drops 226 m over a cliff of sandstone and conglomerate, 251 m including the cascades below, with an average flow of 663 m³ per second, a combination of height and volume that makes it one of the most powerful single-drop waterfalls on Earth.
What the Rocks Built: Islands in the Sky and Canaima National Park
Geology ends in biology here. Each tepui summit is cut off from the forest below by its cliffs, and its climate is cool and wet while the base is hot and humid. The nutrient-poor quartz surface forms almost no humus, which favours carnivorous plants such as sundews (Drosera) and most species of the marsh pitcher Heliamphora, along with many orchids and bromeliads. The Flora of the Venezuelan Guayana, published between 1995 and 2005, recorded 2,447 species of vascular plants in the Pantepui region, 42% of them endemic to the tepuis. Across the whole Guiana Shield, botanists have found 13,367 vascular plant species, about 40% endemic, and more than 3,000 vertebrates. Scientists once assumed all tepui species were relics stranded when the plateau broke apart, but studies of tepui treefrogs suggest some colonised the summits from the lowlands after the tepuis formed.
The highest ground lies at the southern edge, where Pico da Neblina reaches 2,995 m inside Brazil, the highest point in South America east of the Andes. Its massif is a heavily eroded sandstone plateau that has lost the classic tabletop shape. The Yanomami call it Yaripo, "mountain of the winds", and the summit lies within both a national park created in 1979 and the Yanomami Indigenous Territory.
In Venezuela, Canaima National Park was created on 12 June 1962 and listed by UNESCO in 1994; it roughly occupies the same area as the Gran Sabana region and is the second-largest park in Venezuela. Tepuis cover about 65% of its 30,000 km², roughly the size of Belgium. In 2017 the IUCN listed it as an area of significant concern, citing weak protection and management. The word tepui itself comes from the Pemon people of the Gran Sabana and means "house of the gods". Guyana's share of the highlands, and the waterfall that drains it, is covered in the Kaieteur Falls guide.