454 HIGH-LEVEL RIVER TERRACES 



the Zwart Ruggens west- of the Karroo. An obvious 

 possible reason for the cutting of these terraces is that 

 the sea stood higher relatively to the land than is the 

 case to-day, but whether the rivers had previously cut 

 their channels down to sea level and so were unable to 

 deepen further their valleys, or whether the country as 

 a whole sank and therefore checked the deepening of 

 the valleys, is not certain. The former is perhaps the 

 more likely, for no filled-up channels far from the coast 

 have been found, and they might be expected had the 

 valleys been eroded to a greater depth than could be 

 maintained under the new conditions. Local plains 

 might well have been formed behind the larger blocks of 

 mountains, just as we see wide alluvial flats in the course 

 of the Buffel's River behind the Leeuw Kloof Poort 

 and the plain cut by the Olifant's River before entering 

 its gorge in the Gamka Hills. The terraces on the 

 flanks of the mountains, with the outlying table-shaped 

 fragments of the plains that formerly connected them, 

 are so extensively developed, both to the north and 

 south of the Zwartebergen, that they cannot be ex- 

 plained by a local cause like that which is sufficient to 

 account for the alluvial flats of the Olifant's River. It 

 is probable that the high-level plateau in the country 

 south of the Langebergen was formed at the same time 

 as the terraces we have been discussing, although it lies 

 at a lower level, for then, as now, the rivers must have 

 had a fall towards the coast, and each gorge through 

 the great ranges was perhaps more steeply graded than 

 the valley -bottom above or below it. 



The rising of the country relatively to the sea level 



