286 
GEOLOGY IN RURAL WELFARE 
farms most types can be located with safety with regard to water 
supply only by considering fully the conditions of geologic struc¬ 
ture and materials in the vicinity. 
Losses of soil by erosion due to the action of wind or of 
water, and in some localities due to the additional influence of 
improper tillage and pasturage, bear definite relations to the top¬ 
ography of the area effected. Unfortunately the rich, black 
humus of the top-soil, which is the best part of it, is the first to be 
removed — a fact that makes early prevention imperative. If 
the losses are permitted to continue a great succession of gullies 
and barren ravines soon develops, and a worthless area is formed 
where valuable land could have been retained. The water-table 
is perceptibly lowered over large areas by increased depth of drain¬ 
age channels, or removal of protective cover, and this is another 
serious loss. On the other hand, proper drainage may change an 
alkali soil to a fertile one. 
The chief processes that cause these losses involve the princi¬ 
ple that the transporting power of running water varies as the 
sixth-power of its velocity. This means that a current w’hose 
velocity is three miles per hour can carry more than eleven times 
as much sediment as one whose velocity is two miles per hour, 
4 
and that a current of three miles per hour loaded to its capacity will, 
on being reduced to two miles per hour or less, deposit more than 
90 per cent of its load. When a flood current subsides, or is 
checked, an area of rich soil may be covered to a depth of several 
feet with sand and other worthless materials. Prevention and 
partial restoration of losses may be accomplished as follows: 
Meandering channels may be replaced by large drainage ditches 
and, with the aid of catchment basins in regions having high rate 
of rainfall, prevent flooding and erosion of river bottom-land. 
Other losses may be wholly or partly prevented by constructing 
retaining walls, by the use of tiling, or of lined open drains, or by 
contour tillage, by limited pasturage, or by planting trees, shrubs 
or grasses. Restoration may be partly made by constructing dams, 
or by other means of ponding, to check the current and arrest 
the moving sediment, thereby changing the area from one of 
erosion to one of deposition. 
Soil origin finds its explanation chiefly in the field of geology; 
