giant flat-topped mountain, or mesa, in the Pakaraima Mountains of the Guiana Highlands, at the point where the boundaries of Brazil, Venezuela, and Guyana meet. About 9 miles (14 km) long and 9,094 feet (2,772 metres) high, it is the source of many rivers of Guyana, and of the Amazon and Orinoco river systems. Mount Roraima is the highest point in Guyana.