152 THE BOOK OF NATURE STUDY 



The relation of the contouring to canals, roads, and railways 

 should be studied. Canals must adhere to contours, except 

 where locks allow them to go over hills, or where hills are cut 

 through by cuttings or tunnels and valleys bridged. Railways do 

 not often traverse more than one contour interval in two miles, a 

 gradient df i in 100 (Figs. 44 and 48). In this case, too, cuttings, 

 tunnels, and embankments may have to be resorted to. Roads may 

 cross as many as five intervals in a mile a grade of i in 10, lanes 

 and pathways even more. In most cases, however, it will be found 

 that in more important roads the direction has been carefully 

 planned to keep the gradients below i in 12, and to secure this 

 it is often necessary to cross contours obliquely, and even to 

 wind up valleys and back again. In mountainous country like 

 the Alps, to secure uniformity and gentleness of gradient, the 

 system of zigzagging on hill slopes is practised. This is not adapt- 

 able to railways in which a system of corkscrew tunnels must at 

 times be employed. 



Water will flow straight down the steepest slope it can find ; 

 that is, across the contours as directly as possible till it reaches 

 the waterway on a valley floor. It will then still continue to 

 cross the contours as quickly as possible, i.e. from the point of 

 one V to the point bf the next. Thus the direction of drainage in 

 a well-contoured map can be inserted even if the streams have all 

 been omitted (Fig. 44). The grade of streams can be easily worked 

 out, and it will be found usually to be steeper at their heads and 

 to lessen below. The gradient of the valley sides may be either 

 steep and gorge like, or open and gently sloping, and the contours 

 along the sides will be nicked where tributaries or storm torrents 

 come in. Often one side of a valley will be found to be much 

 steeper than the other (Fig. 48). The cross section of a lowland 

 valley will be like a U or a very open V, often with the waterway 

 ill marked and meandering about an almost flat floor. On the 

 other hand, as a stream winds through a valley amongst hills, the 

 curve of the concave bank, where the stream swings against the 

 side of its valley, will be abrupt with crowded contours, while the 

 other side will be gently sloping with widely spread contours 

 (Figs. 44 and 46). 



Plateau ground will show large areas within its upper contours, 



