Ch. 1] EROSION 5 



is dominant in arid areas and in cold regions; chemical weathering 

 prevails in humid, tropic areas. Steep slopes facilitate mechanical 

 erosion; flat areas favor accumulation of water in the rocks and soil 

 and thus help chemical weathering. 



Chemical weathering is essentially a question of exchange and move- 

 ment of ions. Base (cation) exchange is a dominant process. Critical 

 factors are (1) concentration of ions, particularly hydrogen, sodium, 

 calcium, and magnesium; (2) oxidation-reduction potential; and (3) 

 temperature. Basic rocks, that is, rocks rich in calcium and magne- 

 sium and relatively low in silica, tend to be more susceptible to chemi- 

 cal weathering than acid rocks, which are rich in silica and alkalies, 

 but, because the hydrogen-ion concentration of the soil is an im- 

 portant factor, this generalization is subject to exceptions. Many 

 sedimentary rocks weather more rapidly than igneous rocks because 

 of their greater permeability. Moisture and temperature affect the 

 growth of microorganisms, which in turn influence chemical weather- 

 ing. The longer the time that climate, topography, and organic activ- 

 ity remain essentially unchanged for given conditions of runoff, the 

 more thoroughly the rocks are weathered into soil. 



When weathering has proceeded to the state where soil has formed, 

 the earth materials are divided into four distinct zones from the sur- 

 face downward: (1) an upper leached zone relatively rich in organic 

 matter, the A zone; (2) an underlying zone in which some of the ma- 

 terials transported or leached from the upper zone have been deposited, 

 the B zone; (3) a still lower zone of partially weathered rock, the C 

 zone; and (4) essentially unweathered rock. 



If weathering proceeds for a long time, the soil approaches matur- 

 ity, which results in the formation of clay minerals typical of the 

 environment and in the production of many fine particles. The con- 

 stituents of such soils, when transported to a place of deposition, form 

 a sediment having a far different reaction to imposed loads than sedi- 

 ments whose constituents are derived from areas in which the rocks 

 have not developed a chemically mature soil. Students of soil mechan- 

 ics might well consider this point in interpreting the strength of foun- 

 dation materials. 



EROSION 



Both mechanical and chemical processes cause erosion. Because of 

 the close relationship between erosion, transportation, and deposition, 

 it is convenient to discuss the mechanical aspects of these subjects in 

 sequence and then take up chemical weathering and solution. 



Mechanical erosion consists of two processes: (1) plucking or fore- 



