PLANATION SURFACES IN THE KARROO 



397 



quartzite boulders and cobbles from the neighboring Witteberg ridges — a 

 plain of irredeemable barrenness, though not without plant growth. This 

 relation is shown in figure 7. The general location of the terrace is 

 indicated in figure 2. The terrace is evidently the ancient valley floor 

 of the Buffels River system, in the old age of an earlier cycle. The 

 valley floor was broadly opened on an area of moderately resistant 

 strata, after the river had cut down so deeply in the Leeuw Kloof poort 

 of the Witteberg ridge next farther doAvnstream that further deepening 

 became extremely slow. The reduction of the surface to a plain must 

 have been largely aided by the general wasting down of the minor ridges 

 as well as by the lateral swinging of the streams. The distribution of 

 the coarse cobbles over the plain seems to have been the work of sheet- 

 floods, such as become peculiarly effective in the later stages of a cycle of 

 erosion, particularly in a region where occasional heavy downpours of 

 rain occur. The plain is now dissected by Buffels river and its branches, 

 whose valleys have reached a stage of middle or late maturity, at a level 



Figure 7. — Detailed Section of the V'itteberg, Dwyka, and Ecca Series. 



Witteberg strata are on the right, Dwyka in the center, and Ecca on the left, 

 locality is five miles southwest of Laingsburg. 



The 



of from 300 to 500 feet beneath that of the plain. A view of the terrace 

 plain, looking southward toward the ridge from which the cobbles are 

 derived, is given in plate 52, figure 2. A view of the valleys dissected 

 beneath the plain, taken from about the same point as the preceding view, 

 but looking west-northwest, is given in plate 48, figure 1 ; some of the 

 higher knobs in the ridges shown rose, however, above the terrace 

 level. We were told that several other examples of cobble-strewn terraces 

 are known elsewhere among the east-and-west ridges. Some of them 

 have been described by Schwarz. 



The chief reason for mentioning this terrace is to draw attention to 

 its possible explanation by change of climate rather than by change of 

 continental attitude or altitude, the latter explanation having been gener- 

 ally accepted on our excursion. It is well known that a valley floor will 

 be widened by the swinging of a river that has reached the graded condi- 

 tion — that is, the condition of balance between its capacity to do work 

 and the work that it has to do; that the graded condition is one of great 

 delicacy, and that any change in river volume or river load will tend to 



