42 



PHYSICAL GEOLOGY 



Fig. 20. — An erosion pillar, shaped largely 

 by the work of wind-blown sand. Near 

 Adamana, Arizona. 



slopes, since loose material has a tendency to creep down hill 

 (p. 31). In regions which have been covered by glaciers (p. 168) 

 the soil has often been removed from the hilltops by them. 



The same agencies that 

 cause the disintegration of 

 the rock break up the mantle 

 rock to finer and finer par- 

 ticles and form soil. Soil 

 grades into the coarser sub- 

 soil which has not yet been 

 completely disintegrated. 

 This subsoil is gradually 

 brought to the surface by 

 earthworms where the soil is 

 clay, and by ants where it is 

 sandy. In the aggregate, 

 the work of these animals is 

 important. It has been esti- 

 mated that in England earth- 

 worms bring 17 to 18 tons of 

 material an acre to the surface each year, and that in Massachusetts 

 ants bring up one fourth inch of earth. Leaves and other organic 

 matter which are carried into the soil and subsoil by earthworms form 

 organic acids which hasten the chemical disintegration of the rock. 

 Roots of plants and overturned trees also help to mingle soil and sub- 

 soil. The fertility of 

 soil is greatly increased 

 by the organic mat- 

 ter, either animal or 

 vegetable, which it 

 contains, but its char- 

 acter depends largely 

 upon the rock from 

 which it was derived. 



Kinds of Soil. — Mantle rock and soil are moved by hillside creep 

 (p. 31), by rain (p. 33), by avalanches, by landslides (p. 73), by 

 slumping (p. 73), etc. ; all of which combine to remove it from the 

 uplands and carry it to the valleys. There are two kinds of soil, 

 (1) residual soil, that derived from the rock which it covers, such as 

 that which overlies large areas where the country has not been affected 



Fig. 21. — Section showing the thickness of mantle 

 rock on different parts of a hill. (Modified after Cham- 

 berlin.) 



