Guiana Highlands, plateau and low-mountain region of South America located north of the Amazonia and south of the Orinoco River. Comprising a heavily forested plateau, they cover the southern half of Venezuela, all of the Guianas except for the low Atlantic coastal plain, the northern part of Brazil, and a portion of southeastern Colombia. They are geologically similar to the Brazilian Highlands, from which they are separated by the eastern part of the Amazone lowlands. The terrain comprises a mixture of three ascending elements: a basement of rolling hilly upland, standing mostly less than 1,000 feet (300 metres) above sea level; low mountains, near stream divides, ranging from 2,000 to 3,000 feet (600 to 900 metres) above sea level; and tabular plateaus, capped with resistant sandstone. The highest elevations are formed by the tabular plateaus, such as Roraima Massif (9,094 feet [2,772 metres]), where the boundaries of Brazil, Venezuela, and Guyana meet. The Guiana Highlands extend westward across the Upper Orinoco (Britannica).
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| MAP OF THE GUYANA SHIELD AND HIGHLIGHTS THE MASSIFS WITH TEPUI-LIFE-ZONE OF BRAZILIAN TERRITORY |
Below, brief presentations of the three tepuis in Brazilian territory.
NEBLINA MASSIF ‣ represents one of the most interesting sandstone massifs of the Guayana Highlands because of its elevation and extensive summit plateaus between 2000 and 2,993 m; the massif is dissected by a large, deep valley called the Cañon Grande that is drained by the headwaters of the río Mawarinuma. Currently the Venezuelan portion of this region is among the most pristine places on earth, and is a well recognized center of Neotropical plant endemism. Neblina Massif, like the other tabletop sandstone mountains (tepuis) on the Guayana Shield, is formed by a layer of sandstone (the Roraima formation) that lies over an igneous, much older basement dated at ca. 3.5 billion years. For expeditions for this mountains, see Hike and Fly Brasil (LINK).
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| LOCATION, GENERAL APPEARANCE AND SOME LOCATIONS IN NEBLINA MASSIF (FOR ORIGINAL SIZE OPEN IN A NEW TAB) |
ARACÁ MASSIF ‣ located at 00º51' - 57'N and 6º30'14-24' W, approximately 200 km north of the town of Barcelos on the Rio Negro in Amazonas State, Brazil. It is the southernmost extension of the Roraima Formation, consisting of Upper Precambrian quartzitic sandstone with intrusions of volcanic rock, and was formed into its present table mountain shape by the gradual erosion of the softer sandstone rocks. The plateau is at about 1000- 1600 m above sea level and there are steep sandstone cliffs on the southern and western sides, leading to talus slopes that drop down to the sandy plain at 100 m altitude. The northern and eastern sides of Aracá Massif merge into the Curupira Highs, a range of granitic mountains. The highest part of Aracá Massif is a granitic dome that rises from the extreme north of the western part of the plateau to 1600 m. The soil of the plateau is thin, sandy and rocky and acidic. In some areas peat has accumulated and consequently the streams that flow from the plateau are black water. Much of the plateau is waterlogged during the rainy season (from April to November) and quite dry during the dry season (December to March). The two main rivers leave the plateau over spectacular waterfalls with drops of about 700 and 900 m, respectively. Night temperatures on the plateau reached a low of 5°C in February (Prance & Johnson, Kew Bulletin, 1992).
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| LOCATION, GENERAL APPEARANCE AND SOME LOCATIONS IN ARACA MASSIF (FOR ORIGINAL SIZE OPEN IN A NEW TAB) |
RORAIMA MASSIF ‣ Roraima Massif is the highest of the Pakaraima chain of tepuis (table-top mountain) or plateaux in South America. It is located at the junction of Venezuela, Brazil and Guyana. A characteristic large flat-topped mountain surrounded by cliffs 400 to 1,000 meters high. The highest point of Roraima Massif is located on the southern edge of the cliff at an altitude of 2,810 meters in Venezuela, and another protrusion at an altitude of 2,772 meters at the junction of the three countries in the north of the plateau is the highest point in Guyana. The name of 'Roraima' came from the native Pemon people. Roroi in the Pemon language means 'blue-green', and ma means 'great' (Wikipedia).
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| LOCATION, GENERAL APPEARANCE AND SOME LOCATIONS IN RORAIMA MASSIF (FOR ORIGINAL SIZE OPEN IN A NEW TAB) |