138 SCIENCE IN AFRICA 



upper parts and the heavy muds on the lower. The effect of this 

 deposition is often to bury fertile soils on the flat land with infer- 

 tile debris; 3. The increased run-off which follows these processes 

 causes serious torrents which cut gullies or dongas deep into the 

 subsoil. At the same time flooding may lead to destruction in 

 lower parts of the drainage systems; 4. These three processes tend 

 to lower the ground-water table in every part of the country. In 

 addition to the effects of water, erosion by wind is often serious, 

 for in areas where high winds are coincident with the dry season, 

 as in the rift valleys of eastern Africa, pulverized soil is often carried 

 away in the form of dust storms. 



The factors leading to soil erosion can be divided into the 

 natural agencies, which result in the first place in the breaking 

 down of rocks to form soil, and later under special conditions, in 

 a loss of soil quantity and quality; and into human activities, 

 agricultural and other. 



In discussing natural agencies in soil erosion, authorities, especi- 

 ally in South Africa, have emphasized the alleged progressive 

 desiccation of Africa which is supposed to have led, not only to the 

 conversion of wide tracts of fertile country into desert, but also to 

 the deposition of alkali or brak^ over wide areas. This progressive 

 desiccation has been discussed in Chapter IV, where it was pointed 

 out that there has certainly been a great change since the pluvial 

 periods, some 10,000 or. 15,000 years ago, and that this change may 

 still be continuing to-day. Most authorities are now agreed, how- 

 ever, that although reduction in rainfall may have had a slight 

 effect in localized areas, the recent spread of desert conditions 

 w^hich has undoubtedly affected wide areas, especially in South 

 Africa, is due principally to human agencies. 



The effect of human activities depends on the basic principle 

 that w^hen the natural cover of vegetation over the earth is re- 

 moved and the elements are allowed free play, the soil is seriously 

 affected or even washed away bodily. Increases in human and 

 cattle population since the arrival of the white man are held to be 

 largely responsible for the erosion of the African soil. There are, 

 however, areas where the population has remained at a steady 



^ Under arid conditions the subsoil waters are raised near to the surface, and on 

 evaporation deposit their dissolved salts as brak in a form which is often toxic to plant 

 life. When storm rains come they wash away not only the brak but the soil itself. 



