Page:South African Geology - Schwarz - 1912.djvu/98

 Kentani Hill, there are little caps of this material still in place.

The 1500-ft. level is developed near East London and at Sandflats on the Midland line, where marine deposits occur upon it belonging to the uppermost Cretaceous age; elsewhere the plain has been denuded of the deposits and simply exhibits a flat shelf. The marine deposits show that where they occur the plain is a terrace of marine erosion and that the sea beat against the cliffs, now inland, which were topped by the 2500-ft. level.

The 600-ft. peneplain is the general level of the coastal flats right from Caledon, through Swellendam, George, Knysna, Humansdorp, and far to the east. It is covered with great deposits of surface quartzite and ironstone gravel, showing that a part of it, at least, was river-cut, though the seaward end is no doubt due to marine erosion.

Sea-level terraces are well developed off the Bredasdorp and Riversdale coasts, and eastwards along the coast at intervals, as at Port Elizabeth, the Kowie, on the Natal border, and in Zululand. All these terraces swarm with marine organisms, mostly lime-secreting, such as shell fish, Polyzoa, Coralline seaweeds, &c., and regular reefs of newly formed limestone are being constructed which are of the same nature as the coral reefs which begin to make their appearance on the east coast north of Beira, only here the lime-secreting organisms are more entirely coral animals. Such reefs have been called stone reefs in contradistinction to coral reefs; they often contain beach boulders and shore sand.

The —600-ft. terrace, or the shelf submerged 600 ft., is the Agulhas plateau, which prolongs the truncated contour of South Africa some 90 ml. southwards to a