410 



THE NATURE BOOK 



the invading ice the waters were im- 

 pounded as lakes, and thence they escaped 

 across the lower notches of the hills and 

 along devious channels to those parts 

 of the country free 



outer curve the particles have become 

 stretched and are in a state of tension, 

 while the particles of the inner curve 

 have become more closely packed together. 



from ice. 

 retreat of 



On the 

 the ice 



the drainage re- 

 verted, in the main 

 part, to its original 

 system, and the 

 valleys were left 

 dry and function- 

 less. 



So far we have 

 only dealt with 

 valle3'S of erosion 

 carved out of 

 mountain masses 

 b}^ river, glacier, 

 frost and other 

 agent 

 without. 



But there 

 other forces 

 from within, and 

 their function is to 

 crumple the earth's 

 crust into 

 with crests 

 of the sea. 

 the rocks 

 clines. and 

 crests into 

 plementary 



acting from 



are 



actmg 



"THE LATERAL VALLEYS 



LcokinI up the Pennant Valley, 



folds 

 and hollows like the waves 

 In the upraised portions 

 are arched into anti- 

 the strata dip from the 

 the troughs. In the com- 

 hoUows the beds bend 

 into synclines or loops like a hanging 

 chain. We should naturally expect that 

 the arched portions of the folds would 

 exist as mountains, and the synclines as 

 valleys. In the newer mountains and 

 vallej's. such as the Jura mountains, and 

 in the \'alley of the Thames from Reading 

 to the sea, this is the case. But. strange 

 to sav. the reverse is seen in the majority 

 of mountains and valleys. The synclines 

 form the highest peaks, and the \'alleys 

 have an anticlinal structure. A little 

 consideration will show that this is 

 exactly what should happen under normal 

 conditions. 



If we take a bar of india-rubber exactly 

 a foot long and bend it into an arch, 

 we discover on measuring that the con- 

 vex side has extended to more than a 

 foot in length, and the concave side is 

 less than a foot. This means that in the 



WOULD BE LEFT HANGING ON THE 

 SLOPES." 

 Wales. (Hanging Valley marked with an arrow.) 



Apph'ing this to folded rocks less able to 

 accommodate their particles under stress 

 and strain, we find that in anticlines 

 there is a tendency for the strata to crack 

 and produce open joints, while in the 

 synclines they become more compact. 



Thus the fractured and distorted anti- 

 clines readily yield to disintegrating agents, 

 while the more solid and compact syn- 

 clines are better able to resist them. 



DIAGRAMMATIC SECTION THROUGH STRATA 

 WHICH HAVE BEEN FOLDED AND CARVED 

 INTO MOUNTAIN AND VALLEY. 



In course of time the crests become re- 

 duced to the level of the hollows, and the 

 lines of wasting being hrinly established, 

 the action may continue until the former 

 valleys stand up as hills and the mountains 

 have become vallevs. 



