FORMATION OF SOIL 



77 



In the northern portions of the United States the soil is, in most 

 localities, of only moderate depths, because at a late period (geo- 

 logically speaking) this region was covered with a great ice-sheet, 

 which swept away much of the 

 accumulations of ancient rock- 

 decay. In the parts of the coun- 

 try where the ice-sheet did not 

 come, the soil is much deeper, 

 and in tropical lands it attains 

 remarkable depths. In our South- 

 ern States the felspathic rocks are 

 often found thoroughly disinte- 

 grated to depths of 50 or 100 

 feet, while in Brazil the soil is 

 often 200 to 300 feet deep. 



The mechanical effect of rain 

 is less extensive, perhaps, than its 

 chemical work of disintegration, 

 but is very important, neverthe- 

 less. Under ordinary conditions, 

 this mechanical work consists in 

 the washing of soil from higher 

 to lower levels. How consider- 

 able is the movement of soil that 

 has thus been brought about, may 

 be imagined when one sees, after 

 a heavy rain, the rain-rills which 

 run over the slopes, muddy and 



charged with sediments, and how turbid the streams become with 

 the soil which the rain washes into them. Bare soil is rapidly 

 torn up and washed away by the action of rain, but a covering of 

 vegetation, and especially of the elastic and matted stems and 

 roots of grasses, much retards the action. 



Other things being equal, the rapidity with which the rain 

 sweeps away the soil depends upon the steepness of the slope 

 upon which the soil is formed ; for gravity largely determines 



Fig. 22. — Excavation displaying 

 the transition from rock below to soil 

 above. 



