SOILS. 



the fields; the surface of which, apart from the cracks usually 

 formed, may be several inches lower in the dry season than 

 during wet weather. 



, 



RED CLAY SOIL. BLACK " ADOHE " CLAY SOIL. 



FIG. 13. Expansion on Wetting and Contraction on Drying ol heavy clay soi!s. 



Contraction on Wetting. In the case of alkali soils contain- 

 ing much carbonate of soda, a very notable contraction occurs 

 in wetting the loose, dry soil. The cause is here obviously the 

 collapse of the crumbs, formed in dry tillage or crushing, into 

 single grains, closely packed. The same result is observed in 

 the naturally depressed "alkali spots" (see chapt. 22). 



" Hog-wallows." In the field the wetting of cracked clay 

 soils produces some very curious effects. The effect of the 

 first light rains usually is to crumble off the edges or angles 

 near the surface, the materials thus loosened falling into the 

 lower portion of the cracks. This is repeated at each success- 

 ive shower followed by sunshine, the crevices thus becoming 

 partly filled with surface soil. When, subsequently, the heavier 

 and more continuous rains wet the land fully, also causing the 

 consolidated mass in the crevices to expand, the latter cannot- 

 cl6se on account of the surplus material having fallen into 

 them; the result being that the intermediate portions of the soil 

 are compelled to bulge upward, sometimes for six or more 

 inches, creating a very uneven, humpy surface, well-known in 

 the southwestern United States as " hog-wallows," 1 



1 A totally different kind of "hog-wallows,"' occurring in California and the arid 

 region generally, have been described in a previous chapter under the head of 

 Aeolian soils (See chapt. i, p. 9''. 



