PHYSIOGRAPHIC FACTORS 



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tensive. Generally, they occur as talus slopes at the bases of cliffs 

 from which the material has fallen. They are usually potentially 

 good soils because they are mixtures of coarse and fine materials, 

 often originating from several kinds of rocks, and organic matter 

 is likewise mixed with the mineral components. The favorableness 

 of the habitat is primarily determined by the moisture supply, 

 which is strongly variable, depending upon exposure. 



Glacial ice plucks and gouges quantities of soil material from 

 whatever surface it traverses. Carried in the ice, these materials 

 are ground, pulverized, and mixed until they are deposited as 

 moraines at the limit of advance or dropped as the ice recedes. 



FlG. 79. Shrinkage upon drying as illustrated by some Piedmont soils. 

 Samples obtained in place (see Fig. 83), then initially saturated with water 

 and oven-dried. B horizon clays— (1) Orange, (2) White Store, (3) Tirzah; 

 A horizon sandy loam— (4) White Store. Such shrinking and swelling in the 

 B horizon affects soil aeration and water movement.— From Coile. 04 



