ANCXMALOIS STRUCTURAL FORMS. 



L45 



narrow U-form would include the whole course of the river above the 

 initial flood plain. 



Figure 11.— Cross-section of a river Valley having no Flood-plain. 

 Each half of the valley has only two elements, the weather curve a b and the water curve b c. 



Anomalous Valleys of the great Plains. — But certain peculiar conditions 

 sometimes intervene to prevent the development of a Hood plain where 

 it normally belongs. Then we have the anomaly of a valley some miles 

 in breadth without a flood-plain, which is so diametrically opposed to 

 our ordinary conceptions of rivers that we are at once impelled to seek 

 its explanation. It is upon the great plains at the eastern base of the 

 Rocky mountains that we find the most striking examples of wide valleys 

 without flood-plains. The lower part of the water curve (m n, figure 12) 



Figure 12. — Cross-section of a broad Valley of the Plains having no Flood-plain, 

 a l> = weather curve ; b m n = water curve, of which the lower portion, m ??, is greatly extended. 



may be a mile or two in breadth. At the point m the valley wall begins 

 to be well defined. The valley floor m m is also well defined, but quite 

 remarkable for its wide departure from horizontality. The point m may 

 be fifty feet above n. It is unmistakably a valley floor, but its steepness 

 is astonishing and perplexing. Moreover, the anomaly does not stop with 

 the unusual form of the cross-section. The quality of the land contra- 

 dicts all expectations which would naturally be entertained respecting 

 river bottoms. Instead of a uniform stretch of rich alluvium we find 

 irregular alternations of loam, sand, gravel, gumbo and alkali patches. 

 The best element, the loam, may indeed predominate, and hence the 

 valley may support a prosperous agriculture; but the valley lands of the 

 plains are generally inferior to the table lands, thus reversing the condi- 

 tions which usually prevail. 



The plains are built up of incoherent masses of sand, gravel, (day 



