178 
IOWA ACADEMY OF SCIENCE Vor,. XXVII, 1920 
as follows : Meandering channels may be replaced by large drain- 
age 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 con- 
structing retaining walls, by the use of tiling or of lined open 
drains, 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; 
soil distribution, largely in that of physiography. Diflferent kinds 
of soils are produced from different kinds of rock or from the 
same kind of rock when subjected to different processes during the 
course of origin. For example, soils originating from a given 
kind pf rock in a warm, wet climate will be very unlike those 
derived from the same kind of rock in a cool, arid region. A 
third kind of soil will result if the materials from the same kind of 
rock are transported and sorted by water before forming the 
final soil; a fourth kind if transported by glaciation; and a fifth, 
if deposited by the wind. The various kinds of soil may differ 
from each other in number of mineral constituents or in the 
different proportions of each. The development of hills and 
valleys and other topographic forms by erosion gives rise to a 
different kind of soil in each topographic location. Kinds of soil 
arise also in numerous other ways each of which is a response 
either directly or indirectly to geologic or physiographic processes 
and conditions. 
Classification of soils that they may be subjected to treatment 
conducive to the greatest production depends chiefly on the accu- 
rate use of the principles of soil origin and distribution. The 
changes recently made by the United States Bureau of Soils in the 
revision of classification units that were used in mapping a number 
of years ago afford excellent illustrations of this fact and of its 
recognition by the Soil Survey. The new divisions formed are 
based almost wholly on genetic and topographic relations- — the 
principles of geology and physiography being applied to a much 
greater extent and in greater detail than in the earlier work. 
The distribution of vegetation in so far as it is controlled by 
topography, kind of rock and geologic structure constitutes an 
important phase of agricultural geology. The distribution of soils, 
of rainfall, of temperature and of plant and animal life, the lo- 
