46 NATUEE AND PBOPERTIES OF SOILS 



agriculturally because of tlieir small area, their inaccessibility, 

 and their unfavorable physical and chemical characteristics. 



28. Alluvial soils.^— In considering water as a soil-form- 

 ing agency, it was found to have both cutting and transporting 

 powers. Alluvial soils are the direct result of these activities, 

 especially the latter. The carrying power of water varies di- 

 rectly as the sixth power of the velocity ; so that doubling the 

 velocity increases the transportive ability sixty-four times. 

 Obviously any checking of a stream's velocity will force it to 

 deposit its load, the larger particles first and the finer as the 

 ciu-rent becomes more sluggish. With changes of velocity dif- 

 ferent grades of material are laid down, giving rise to strati- 

 fication, one of the important characteristics of an alluvial 

 soil. Streams never deposit, either along their course or at 

 their delta, all of their sediment. Many tons of material both 

 in suspension and in solution are discharged yearly into the 

 oeean.^ 



There are three general classes of alluvial soils: (1) flood 

 plain deposits, (2) deltas, and (3) alluvial fans. As the 

 outlet of a stream is approached, its gradient generally be- 

 comes less inclined and its current is slackened. In a large 

 stream this often means an aggrading of the channel due to 

 the deposited material. A stream on a gently inclined bed 

 usually begins to swing from side to side in long, gentle 

 curves, depositing alluvial material on the inside of the curves 

 and cutting on the opposite banks. This results in oxbows and 



^ If a detailed discussion regarding alluvial, marine, and glacial activi- 

 ties is desired, the following books will be helpful: Tarr, B. S., and 

 Martin, h. M., College Physiography; New York, 1918. Pirsson, L. V., 

 A Text Boole of Geology, Part I; New York, 1915. Emerson, H. L., 

 Agricultural Geology; New York, 1920. The considerations of river 

 and stream action by Eussell and Bavis are classical: Eussell, I. C, 

 Mmrs of North America; New York, 1898. Davis, W. M., The Bivers 

 and Valleys of Fenjisyluania; Geological Essays, Boston, 1909. 



*;Streams each year discharge into the ocean, on the average, 100 tons 

 of soluble matter for every square mile of drainage area. The Mississippi 

 Eiver pours into the Gulf of Mexico each year 406,250,000 tons of sedi- 

 ment, not to mention a vast amount of soluble salts. 



