been broken down and ^ 

 the over-lying soil de- 

 rived from them. 



70. Influence of Rock 

 Fissures. An examina- 

 tion of almost any quar- 

 ry where considerable 

 surfaces are exposed re- 

 veals the presence of 

 systems of fissures which 

 divide the stone layers 

 into blocks of various 

 sizes and at the same 

 time provide easy ave- 

 nues for the entrance of 

 surface waters. These 

 features are shown 



clearly in Figs. 10, 11, FIG lO.-Fort Danger, Wis., showing rock fls- 



12 and 13, and into felires wnich Fead to rock destruction, 

 them the roots of trees 



(After Chamberlin.) 



\ 



sometimes make their 

 way where by expansion, 

 due to growth, such strong 

 pressures are developed 

 as sometimes to throw 

 down large blocks of 

 stone. Then again, in 

 cold climates these fis- 

 sures may become filled 

 with water which, when 

 freezing, overturns and 

 throws down many frag- 

 ments, thus hastening 

 their passage into soil. 



71. Soil Removal. It 

 follows from what has 

 been said that the same 

 processes which result in 



FIG 11 -Bee Bluff, Wis., showing rock fissures r ., f ,. ^ 



which lead to rock destruction. (After SOI 1 formation must aiSO 



contribute to its destmc- 



