Passing from the mountains to the great plateau they enclose, the first point to be noted is that its surface is set at two opposite "tilts," the portion north of the Witwaters Rand inclining downward to the east, the other, south of that ridge, to the west. The drainage, therefore, runs respectively east and west, and it is effected by the two great streams of the Limpopo and the Orange, with their many affluents. The general river system of the central plains is thus of the simplest; the Indian Ocean receives their northern waters, the Atlantic their southern; the remarkable factor of the arrangement being that a physical feature so insignificant as the Witwaters Rand should perform the function of water-parting for a region so gigantic.[64] [Footnote 64: There are, of course, in South Africa numerous minor and local watersheds (_e.g._, the Drakensberg, where they initiate the drainage of Natal in an easterly direction, and the mountains of southern Cape Colony, which send some of her rivers southward to the Indian Ocean). These have been necessarily almost disregarded in so general a survey of the sub-continent as that aimed at in the present chapter.] [Sidenote: Course of Limpopo.] The Limpopo, or Crocodile river, rises as a paltry stream in the Witwaters Rand between Johannesburg and Pretoria, and flows into the Indian Ocean, 80 miles north of Delagoa Bay, covering in its course fully 1,350 miles. [Sidenote: Course of Orange.] The Orange has three distinct sets of headstreams from the western flank of the Drakensberg, and a total length of 1,300 miles. From the Basuto border to Ramah, on the Kimberley railway, about 220 miles, it divided the Orange Free State from Cape Colony. The Orange receives on its right bank its greatest affluent, the Vaal, which is between 500 and 600 miles in length. Commercially, both the Orange and the Vaal are as useless as their smallest tributary, being entirely unnavigable at all times of the year. Raging floods in the wet season, and mere driblets in the dry, they are at present denied to the most powerful or shallowest of river steamboats. The prospects of the Orange river as a potential waterway are in any case practically destroyed by a great bar which blocks approach to the estuary from the sea. [Sidenote: Military character of streams of S.A.]