224 



EARTH FEATURES AND THEIR MEANING 



such large measure of frostwork as is characteristic of the sub- 

 polar regions or of high levels in the arid lands. In humid regions 



the rocks become a prey espe- 

 cially to the processes of solu- 

 tion and accompanying chemi- 

 cal decomposition, and these 

 processes, although guided by 

 the course of the percolating 

 ground water along the frac- 

 ture planes, do not afford such 

 striking examples of the con- 

 trol of surface relief. 



Those limestones which 

 slowly pass into solution in 

 the percolating water do, how- 

 ever, quite generally indicate 

 a localization of the solution 

 along the joint channels (Fig. 



FIG. 238. Rain sculpturing under control 239 and plate 6 B). Though 

 by joints. Coast of southern California in other rocks not SO apparent, 

 (after a photograph by Fairbanks). yet golutions gene rally take 



their courses along the same channels, and upon them is localized 

 the development of the newly formed hydrated and carbonate 

 minerals, as is well illustrated by 

 the phenomenon of spheroidal 

 weathering (Fig. 155, p. 150). 



The fracture control of the drain- 

 age lines. The etching out of 

 the earth's architectural plan in 

 the surface relief, which we have 

 seen begun in the processes of 

 weathering, is continued after the 

 transporting agents have become Fl - 23 9.-- Outcrop of flaggy limestone 



. . which shows the effects of solution 



effective. It IS often easy, to See along neighboring joints in a sagging 



that a river has taken its course of the u PP er beds (after Gilbert, U. 

 in rectangular zigzags like the S ' G ' S ' } ' 



elbows of a jointed stove pipe, and that its walls are formed 

 of joint planes from which an occasional squared buttress pro- 

 jects into the channel. This structure is rendered in the plan of 



