144 FARM DEVELOPMENT 



sidewise, it naturally seeks its lowest level and follows 

 the slope of the layer of clay or stone. If this imper- 

 vious layer extends out to the side of a hill, the water 

 flows out and spreads through the surface soil of the 

 hillside. Since this flow of spring water is more or less 

 constant, it may keep a considerable layer of the surface 

 of the hillside or level land beyond the hill, or of a de- 

 pression into which it runs, so wet that there is too 

 much water in the soil for cultivated plants, and only 

 sedges and other water-loving plants will grow. If 

 there is considerable of this water centered in one point, 

 we term it a spring. In some cases the spring water 

 oozes slowly out over a wide area ; in other cases it 

 flows gently from one place; and in others it bubbles 

 upward as if confined between an upper and a lower im- 

 pervious stratum and had broken a passageway through 

 the upper one, and thus finding an outlet had centered 

 in a spring. Some springy hillsides have been so long 

 kept thoroughly saturated with water that the dead 

 roots, stems and leaves of plants have been preserved 

 and a layer of peat has been formed. 



Flat lands. Lands which have not a natural slope are 

 often kept wet by more rain falling on them than runs 

 off or is evaporated. Thus, in Louisiana, a large area of 

 land, made up of deposits from the Mississippi river, is 

 flat and must be drained to be adapted to the growth of 

 cultivated crops. In the valley of the Red River of 

 the North, in Minnesota, North Dakota and Manitoba, 

 likewise, there is a large level area formed by deposits 

 of coarse till and on top of this a fine clay from the 

 great glacier. This was deposited while that area was 

 covered by what is now known as " Ancient Lake 

 Agassiz." (See Figure 7.) In Illinois, Indiana and 

 Iowa, there are large level areas from which the natural 

 rainfall is not removed with sufficient rapidity by natural 

 drainage and evaporation to make them suitable for the 



