144 L. E. HICKS — SOME ELEMENTS OF I. AND SCULPTURE. 



form a gumbo * which imparts the impervious quality to the subsoil, and 

 at the same time their small bulk and relatively slow growth occasions a 

 line of depression. Even when well drained, this part of the river bottom 

 is apt to be cold and sour mitil redeemed by tillage and the admixture 

 of silicious elements. 



True Form <;/' Cross-section. — The old definition of the upper course of a 

 river postulates a V-shaped cross-section.. This is not strictly true, even 

 of fresh ravines at the headwaters. The sides are water curves of erosion, 

 and they conform to the general law that the slope varies inversely as 

 the quantity of water flowing upon it. Since the bottom of a slope re- 

 ceives the water from above as well as its own quota of the rainfall, its 

 pitch must always be less steep than that above if it is a true water 

 curve. One side of a ravine may be ;i talus with its sharp structural 

 angle and uniform declivity, the corrasion at its base proceeding so rap- 

 idly as to give no time for a water curve to form. Very rarely, when the 

 bottom only of a ravine is undergoing erosion, a talus slope may exist on 

 both sides, and the section will then be accurately V-shaped; but if the 

 corrasion at the base of either wall lags or stops, the wash of that slope 

 will speedily convert it into a concave water curve of erosion. The actual 

 result is usually composite, including elements or remnants of a structural 

 angle, modified by a water curve. At all events, it is a curve rather than 

 an angle, and a V with curved sides quickly passes into a U. The middle 

 course of a river is usually said to have a U-shaped valley, hut in fact 

 this form belongs to all parts of a river, the onl}' difference being the 

 breadth of the U. It is in the upper course alone that it has anything 

 like normal proportions. In the middle, and especially in the lower 

 course, it sprawls out and flattens down until all resemblance to the fifth 

 vowel is lost. Moreover, the presence of terraces and water curves of 

 deposition so complicates the cro<s-section that the comparison of it with 

 any letter of the alphabet is no longer useful. 



Cross-section of River having m> Flood-plain. — The most important real 

 distinction between the different parts of a river valley is the presence or 

 absence of a flood-plain. If this is present the section will he as in figure 

 10; if it is absent, it will be as in figure 11. in which, as compared with 

 figure 10, the middle portion, including the flood-plain with its convex 

 curvature, has been cut out. 



There is a wide range of variation among different rivers in that por- 

 tion of the valley which has a cross-section like figure 11. It may he 

 very narrowly U-shaped, or it may open out to such breadth as to he- 

 come anomalous. Under ordinary climatic and geologic conditions the 



*Gumbo is a peculiar, tenacious, fine-grained clay. The use .-mil meaning of this term in the 

 western states is so well established, that it promises t" become a useful and expressive word. ' 



