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NATURE STUDY REVIEW 



[9 :3— Mar., 1913 



comes clear to the student as he continues his field work. It 

 remains for him to determine what these deposits are, and their 

 characteristics. On the gentle gradient at the mouth of the gully 

 he finds a fan-shaped deposit extending away from the gully's 

 mouth (1) with a comparatively uniform slope, (2) with the 

 coarsest material at its apex and the finest around its outer margin, 

 and (3) with numerous little stream channels extending from 

 tlie apex to all parts of the fan. He notes that the fan-shape 

 M due to the shifting of the temporary streams from one side 

 IP the deposit to the other. Why do they shift? An application 

 W§ what he has learned makes it clear that a single rain shower 

 may build up one side so high that the water from the next rain 

 will go down the other side. This deposit is an alluvial fan. 



At the mouth of the stream, where the current is checked by 

 coming in contact with quiet water, he finds a deposit of fine 

 alluvium. This is the delta. Above water it resembles the alluvial 

 fan but a few soundings off its outer edge will show it to have 

 a steep front slope. Usually it is composed of finer sediment 

 than the fan. Miniature deltas may be found frequently in shal- 

 low pools formed during a rain shower. 



Figure 2. — Natural levee along Thorn Creek, near Chicago, 111. 



