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PHYSICAL GEOLOGY 



accomplished by (i) the mere impact of the water, especially if the 

 rock is easily disintegrated (Fig. 60). The effect of clear water upon 

 striking loose sediment with great force is well shown in hydraulic 

 mining. This principle was also employed in the leveling of a portion 

 of Seattle, where a high hill was cut down by means of a power- 

 ful stream of water. (2) In thinly bedded rocks, such as shales 



Fig. 60. — A bank undercut by clear water. (U. S. Geol. Surv.) 



(p. 250), the stream bed may be deepened by " lifting " ; that is, the 

 shale, broken by joints, is separated by the water along the bedding 

 planes (p. 234) ; and the fragments are thus floated off. The 

 effect of this process alone in regions underlain by shales may be 

 of the greatest importance. "Lifting" is especially effective when 

 the stream beds have been exposed to the weather at low water. 

 At such times, temperature changes or frost may loosen much 

 material in the bed, which is picked up and removed during high 

 water. Water without sediment has little effect in eroding thick 



rub) and abrasion are used as synonyms, meaning the detachment of rock particles as 

 a result of wear; corrosion (Latin, COffodere, to gnaw) is used for the work done by solution; 

 erosion is used to ini hide both corrasion and corrosion, as when we say a river erodes its valley, 



l rode it bore The term denudation is reserved for the lowering of a land surface by 



any agent jr. 



