Most of the larger rivers of the state have their sources among the mountains or on the Alleghany Plateau and flow more or less circuitously in a general north-western direction into the Ohio.
West of the Cordillera Oriental, where special conditions prevail, a great desert plateau stretches entirely across one corner of the republic. Apart from the Andean system there is a group of low, broken, gneiss ranges stretching along the east side of Bolivia among the upper affluents of the Mamore and Guapore, which appear to belong to the older Brazilian orographic system, from which they have been separated by the erosive action of water.
The general elevation of the plateau is estimated to be about 2700 ft., and the highest elevation was reported in 1892 to be the Serra dos Pyreneos (5250 ft.).
The disturbances among the underlying rocks of Ohio have been slight, and originally the surface was a plain only slightly undulating; stream dissection changed the region to one of numberless hills and valleys; glacial drift then filled up the valleys over large broken areas, forming the remarkably level till plains of northwestern Ohio; but at the same time other areas were broken by the uneven distribution of the drift, and south-eastern Ohio, which was unglaciated, retains its rugged hilly character, gradually merging with the typical plateau country farther S.E.
The remainder and greater part of the county is occupied chiefly by the gently inclined Old Red Sandstone; in the dissected plateau of the Black Mountains north of Crickhowell the lower marls and cornstones are laid open, while south of Brecon the conglomeratic upper beds form the escarpment and plateaus of the Beacons.
But the Plateau was to be reached by Shackleton's way up the Beardmore glacier at which point the tracks converged.
Diamantina is at present the most important district; it occupies a mountainous plateau, and the diamonds are found both on the plateau and in the river valleys below it.
Adrar occupies the most elevated part of a plateau which ends westwards in a steep escarpment and falls to the east in a succession of steps.
Phoenicia and the Lebanon have the densest population, over 70 to the square mile, while Palestine, the north part of the western plateau east of Jordan, the oases of Damascus and Aleppo, the Orontes valley, and parts of Cornmagene, are well peopled.
The Gongola, flowing east and south to the Benue, rises in the Sura district, and from the Kibyen plateau streams flow north to Lake Chad, west to the Kaduna, and south to the Benue.