CHAPTER XVII 

 REPEATING PATTERNS IN THE EARTH RELIEF 



The weathering processes under control of the fracture system. 

 In an earlier chapter it was learned that the rocks which com- 

 pose the earth's surface shell are intersected by a system of 

 joint fractures which in little-disturbed areas divide the surface 

 beds into nearly square perpendicular prisms (Fig. 36, p. 55), 

 more or less modified by additional diagonal joints, and often 

 also by more disorderly fractures. Throughout large areas these 

 fractures may maintain nearly constant directions, though either 

 one or more of the master series may be locally absent. This 

 distinctive architecture of the surface shell of the lithosphere has 

 exercised its influence upon the various weathering processes, as it 

 has also upon the activities of running water and of other less 

 common transporting agencies at the surface. 



Within high latitudes, where frost action is the dominant 

 weathering process, the water, by insinuating itself along the 

 joints and through repeated freezings, has broken down the rock 

 in the immediate neighborhood of these fractures, and so has 

 impressed upon the surface an image of the underlying pattern 

 of structure lines (plate 10 A). 



In much lower latitudes and in regions of insufficient rainfall, 

 the same structures are impressed upon the relief, but by other 

 weathering processes. In the case of the less coherent deposits 

 in these provinces, the initial forms of their erosional surface have 

 sometimes been determined by the dash of rain from the sudden 

 cloudburst. Thus the " bad lands " may have their initial gullies 

 directed and spaced in conformity with the underlying joint struc- 

 tures (Fig. 238). 



In such portions of the temperate regions as are favored by a 

 humid climate, the mat of vegetation holds down a layer of soil, 

 and mat and soil in cooperation are effective in preventing any 



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