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The Guiana Shield〔The term ''Guiana'' or ''The Guianas'' is often used as a collective name for Guyana, Suriname and French Guiana, and sometimes even includes the portions of Colombia, Venezuela and Brazil which are on the Guiana Shield.〕 is one of the three cratons of the South American Plate. It is a 1.7 billion-year-old Precambrian geological formation in northeast South America that forms a portion of the northern coast. The higher elevations on the shield are called the Guiana Highlands, which is where the table-like mountains called tepuis are found. The Guiana Highlands are also the source of some of the world's most spectacular waterfalls such as Angel Falls, Kaieteur Falls and Kuquenan Falls. The Guiana Shield underlies Guyana (previously British Guiana), Suriname (previously Dutch Guiana) and French Guiana (or Guyane), as well as parts of Colombia, Venezuela and Brazil. The rocks of the Guiana Shield consist of metasediments and metavolcanics (greenstones) overlain by quasi-horizontal layers of sandstones, quartzites, shales and conglomerates intruded by sills of younger mafic intrusives such as gabbros. ==Geomorphology== There are three upland areas of the Guiana Shield: # The Guiana Highlands proper are in Venezuela east of the Orinoco and extend across much of west-central Guyana and into the northern Roraima state in Brazil. # The Tumucumaque Uplands which are a series of central massifs in an arc from the Wilhelmina Mountains of south-central Suriname, along the southern boundary of Suriname and Guyana, forming the Acarai Mountains of Roraima state and the Tumuc-Humac Mountains of Pará and Amapá states of Brazil. From this arc, the southern uplands slope gently downwards towards the Amazon River and the northern uplands slope gently downwards toward the Atlantic. # The Chiribiquete Plateau is a sandstone topped plateau with an elevation of that forms the western edge of the shield. The plateau is separated from the eastern Andes by the thick Neogene sediments of the Sub-Andean Trough that runs along the northern and western rim of the Guiana Shield. The north-central part of the Guiana Highlands is dominated by high flat-topped peaks called tepuis, of the Roraima supergroup and Quasi-Roraima formation, and the rounded granite peaks of the Parguaza and Imataca complexes to the north and southwestern edges of the area. The highest point in the shield is Pico da Neblina in Brazil at 2,994 m (9,822 ft). Pico da Neblina is the highest summit of the larger Neblina massif, a highly eroded sandstone plateau that straddles the Venezuela-Brazil border and that has lost the typical tabletop shape of the other tepuis in the region. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Guiana Shield」の詳細全文を読む スポンサード リンク
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