Title: Bulletin of the Geological Society of America
Identifier: bulletinofgeolo221911geol (find matches)
Year: 1890 (1890s)
Authors: Geological Society of America
Subjects: Geology
Publisher: (New York : The Society)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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138 W. H. HOBBSâREPEATING 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 .^'â 'f's'ii
Text Appearing After Image:
Figure 15.âSagging 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.

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