Formation of Soil. 



55 



it is easy to note how rapidly soil and sand grains are be- 

 ing rolled and tumbled along the bottom. If it is desired 

 to measure this rate of movement a shallow pan or box 

 may be sunk in the 

 bed of the stream, 

 leaving its rim flush 

 with the surface over 

 which the water rolls. 

 After a sufficient in- 

 terval remove the box 

 and dry and weigh 

 the material collected. 

 At each bend in a 

 stream soil is being 

 taken from the con- 

 cave side and carried 

 onward toward the 

 sea, while on the op- 

 posite side new soil is 

 being formed from 

 that dragged along 

 the bottom. In this 

 manner streams 

 change their courses 



j __ j r -\ FIG. 13. Pillar Rock, Wis., showing rocky cliff 



and Wander Irom Side i? the last stages of decay. (After Cbamber- 



to side across the val- lm<) 



ley, each time making a new soil on the side from which 

 they are retreating and carrying away an older soil from 

 ncroaching sido. It is in this way that broad and 

 flat river valleys are formed, with their terraces, such as 

 are shown in Fig. 14. It is in this way, too, that the "ox- 

 bows" of the Mississippi below Yicksburg were formed, 

 some of which are represented in Fig. 15. 



These abandoned river channels are at first long and 

 narrow lakes but ultimately, with the repeated overflows 

 of the stream, they become filled. Sometimes they remain 

 for long intervals depressions in which swamp or humus 

 soils develop. 



