138 W. H. HOBBS — REPEATING PATTERNS IN STRUCTURE OF LAND 



clearness the composite blocks of larger order, is represented by plate 12, 

 figure 1. These structures are developed in deposits laid down behind 

 an old irrigation dam in the Syrian desert. 



Where in desert regions the action of frost becomes especially impor- 

 tant, as on the Tian Shan Plateau, photographs show particularly well the 

 controlling influence of the joint planes in the fashioning of the relief 

 (see plate 10, figure 1). Here the larger sags in the horizon line are 

 above spaced clefts, with smaller subequal intervals separating consecutive 

 joints. 



In the Kalihari of Africa are almost featureless plains which range 

 through twenty degrees of longitude. In a part of this district the domi- 



^' * -^M 



.^'■'f's'ii 



Figure 15. — Sagging of Beds of thin, flaggy Limestone over Joints 

 (After Gilbert, U. S. Geological Survey) 



nant influence of a system of joints and faults in fixing the lines of 

 drainage is well brought out by the Batoka gorge of the Zambesi Eiver 

 below the famous Victoria Falls. The gorge is here directed in largely 

 rectangular zigzags, at the turnings of which are remarkable ^^elbows^' 

 where tributary streams enter the main gorge (see figure 14). Lamplugh 

 has shown that the erosive work of the stream and its tendency to utilize 

 the existing structure lines are much increased by the cloud-burst type 

 of precipitation so characteristic of semi-arid regions.^^ . 



Within relatively humid regions the control of relief by fracture planes 



1' G. W. Lamplugh : The geology of the Zambesi basin around the Batoka gorge (Rho- 

 desia). Quarterly Journal of the Geological Society, vol. 63, 1907, pp. 187-195, figs. 4-8. 



