PHYSIOGRAPHIC FACTORS 



149 



tion of hard, insoluble material like quartz, its soils will be sandy 

 or even coarser. 



Cumulose materials may be mixed with mineral soils in any 

 proportion or may have accumulated as almost pure organic 

 masses. The latter are illustrated by peat bogs, which are common- 



FlG. 77. A wide flood plain in an old river valley whose alluvial soils con- 

 stitute the best farming land in the region. Hiawassee River, Tenn— U. S. 

 Forest Service. 



ly made up of plant remains that only partially decayed and were 

 added to year after year until the lake or pond in which they grew 

 was completely filled. Found most abundantly in lakes produced 

 by glacial topography, the peat accumulations are likewise great- 

 est where temperatures are low enough to limit the activities of 

 organisms that produce decay. 



Transported Soils— On the great part of the earth's surface 

 covered with residual soil, the effects of transporting agents are 

 commonly noticeable only locally. But, to the ecologist, these lo- 

 calities are of interest because the soil conditions are usually differ- 

 ent enough to cause vegetational differences too. 



Except for loess, discussed elsewhere (p. 112), soils of aeolian 

 origin are usually sandy deposits, which wind picked up from 

 wide exposed beaches of lakes or oceans. Normally occurring as 

 dunes, they usually form unfavorable habitats because of the low 

 water-holding ability of sand, its relative sterility, and because the 



