150 THE STUDY OF PLANT COMMUNITIES * Chapter VII 



dunes are subject to blowouts should the surface cover of vegeta- 

 tion be incomplete (see Figs. 55, 56). In contrast, stabilized dunes 

 of arid or semiarid regions form relatively favorable habitats be- 

 cause almost all the water that falls upon them is available for 

 plant use. 



Alluvial soils have been deposited by streams, which, as trans- 

 porting agents, are effective in proportion to their velocity and 

 the size of particles involved. Since currents are rarely constant, 

 the size of transported particles varies, and deposits are always 

 noticeably stratified. Alluvial soils are characteristic of lowlands 

 that formed as deltas in or at the mouths of streams or as flood 

 plains along streams that periodically overflow their banks. The 

 greater the distance from the main channel of the stream, the finer 

 the texture of the soil materials deposited. Alluvial deposits usually 

 make desirable agricultural land if properly drained, and, because 

 of favorable moisture conditions, they usually support the richest 

 natural flora of a region. 



Colluvial materials are transported by gravity. Except in regions 

 of rugged topography or in mountainous areas, they are rarely ex- 



FlG. 78. Colluvial cones, still in formation in Colorado. Only in such 

 rugged mountain topography is gravity of direct significance in soil trans- 

 port.— U. S. Forest Service. 



