GEOLOGICAL CLASSIFICATION OF SOILS 41 



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ideal for the deposition of alluvial matter. Deltas are 

 another good example of alluvial deposits, whether oc- 

 curring in ocean, gulf, or lake. Due to a change in grade, 

 a stream may cut down through its already well-formed 

 alluvial deposit, leaving terraces on one or both sides. 

 Often two, or even three, terraces may be detected along a 

 valley, marking a time when the stream bed was at 

 these elevations. On the lower slopes of hills border- 

 ing valleys, the colluvial deposits may touch or even 

 mingle with the alluvial, and furnish a stream with some 

 of its detritus. 



Alluvial soils, then, are found as narrow ribbons along 

 streams. They are always young soils, and are still in 

 process of formation. Since in most cases they are de- 

 posited by slowly moving water, the texture of such soils 

 is fine, the soils being mostly clays, silts, and fine sandy 

 loams. Found in low lands, alluvial soils need drainage 

 to a large extent. Because of the favorable moisture con- 

 ditions these soils usually have a very large amount of 

 organic material, as vegetation grows readily under such 

 conditions. Considerable humus is also washed into 

 alluvial materials at the time of their deposition. The 

 soil is usually deep, and, because of the high organic con- 

 tent, universally of good physical condition, although 

 very heavy stiff clays may be found in certain cases. 

 The character of the soils and the rocks from which the 

 detritus has been obtained exerts considerable influence 

 on its character. For example, a red soil will often 

 give rise to a reddish alluvial soil, while a soil or a rock 

 poor in lime will certainly not be parent to a soil very 

 much richer in that constituent. 



30. Distribution of alluvial soils. — The distribution 

 of alluvial soils in the United States is not wide, although 



