SOIL-FOEMING PROCESSES 21 



tliat the wind is of especial importance in soil formation. The 

 movement of sand and dust in both humid and arid regions 

 is almost incessant. In desert storms 200 tons of materials 

 have been known to float over every acre of land. The finer 

 particles travel for miles in a very short time. Southern Italy 

 has received as much as one inch of dust from Africa during 

 a single storm. The movement of sand dunes is but another 

 evidence of the transporting power of air in motion. 



Wind as an agency in soil formation would perhaps receive 

 much less attention were it not for the existence of large 

 areas of a certain silty soil called loess. This soil exists along 

 the Rhine both in France and Germany, in southern Russia, 

 in Roumania, in China and in central United States. This 

 material, as well as the adohe of our arid Southwest is con- 

 sidered as largely wind laid. Since the loess is highly fertile 

 and of great agricultural importance, added attention is thus 

 directed towards wind as a soil-forming agency. (See Fig. 4.) 



14. Change in temperature. — Variations of temperature, 

 especially if sudden or wide, greatly augment the denuding 

 actions of water, ice, and wind. Rocks and soil become heated 

 during the day and at night often cool much below the tem- 

 perature of the air. This warming and cooling is particularly 

 effective as a disintegrating agent. Rocks are mineral aggre- 

 gates, the minerals varying in their coefficients of expansion. 

 With every temperature change differential stresses are set 

 up, which eventually must produce cracks and rifts, since the 

 minerals never assume their original position. Incipient focii 

 for further physical and chemical change are thus established. 

 Although the expansion coefficient of rock is low, it must be 

 remembered that very large surfaces are involved. Moreover, 

 it is the multiplicity of the rifts rather than their magnitude 

 that is important. 



The influence of temperature change is manifested on rocks 

 in another way. Due to slow conduction the outer surface 

 of a rock often maintains a markedly different temperature 



