PENEPLANATION OF THE PLATEAU 311 



chiefly by deflation and sheetflood work. The first cycle, represented by 

 the higlier portions of the plateau and the summits of the higher monad- 

 nocks, is being developed by normal methods of erosion. All of the 

 debris carried from the first cycle surfaces must finally be deposited on 

 the second cycle surfaces below; hence degradation of the first cycle can 

 not go on more rapidly tlian conditions on the second cycle surface will 

 permit. If the second cycle surface is being rapidly degraded by the 

 agencies operating there, then opportunity is furnished for the carriage 

 of more debris from the first cycle surface and it is swept down readily, 

 and degradation is thus accelerated on the first cycle surface in turn. 

 The reverse is true if degradation is inactive on the second cycle floor. 



The present topography of the semi-arid belt indicates forcefully that 

 peneplanation takes place at high altitudes without the development of 

 great relief. This type of peneplanation has been called an arid type, 

 but tlie conditions in East Airiesi show that it may take place where 

 rainfall is fairly abundant, provided it comes in widely separated seasons. 

 Aridity is perhaps the great factor, but the conception of aridity must 

 be broadened to include not only conditions where rainfall is scanty, but 

 also those where it comes seasonally; and, under the latter case, where 

 it falls in short showers with sufiRcient intervals to permit the soil to dry 

 out between showers. Aridity the year around and aridity for a part of 

 the year each will produce this type of penei:)lanation. It is not solely 

 a question of the amount of rainfall, but of its distribution as well, and 

 especially is it a question of its concentration within the limits of a few 

 weeks. Peneplanation of the type here discussed will go on in any 

 region, just as it apj)arently is doing in East Africa, provided there is a 

 margin of arid conditions over humid conditions year in and year out. 

 This type of peneplain should be looked for, then, not only in arid re- 

 gions, but also in any region where there are short seasonal rains alter- 

 nating with more extended dry seasons. 



The method of peneplanation in East Africa calls attention afresh to 

 the dangers attending current interpretations of this phenomenon. For 

 instance, suppose that the climate of East Africa were to change in such 

 a way that rainfall became abundant : the present intermittent streams 

 would become permanent, the present wide, shallow valleys would be 

 deepened greatly, and there would be a system of deep, youthful valleys 

 developed in a peneplained upland. Generally speaking, current opinion 

 would interpret this condition as resulting from a reduction of the region 

 to baselevel, then a subsequent uplift, which rejuvenated the streams 

 and developed a new cycle in the older one. It is evident that the whole 

 phenomena could be brought about simply by a relatively snuill change 



