HIGHWAY CONSTRUCTION 13 



diate bank of a large stream must of necessity intersect all the tribu- 

 taries confluent on that bank, thereby demanding a corresponding 

 number of bridges. Those, again, which are situated along the 

 slopes of hills are more liable in rainy weather to suffer from washing 

 away of the earthwork and sliding of the embankments; the others 

 w r hich are placed in valleys or elevated plateaux, when the line crosses 

 the ridges dividing the principal water courses will have steep ascents 

 and descents. 



In making an examination of a tract of country, the first point 

 to attract notice is the unevenness or undulations of its surface, which 

 appears to be entirely without system, order, or arrangement; but 

 upon closer examination it will be perceived that one general prin- 

 ciple of configuration obtains even in the most irregular countries. 

 The country is intersected in various directions by main water courses 

 or rivers, which increase in size as they approach the point of their 

 discharge. Towards these main rivers lesser rivers approach on 

 both sides, running right and left through the country, and into these, 

 again, enter still smaller streams and brooks. The streams thus 

 divide the hills into branches or spurs having approximately the same 

 direction as themselves, and the ground falls in every direction from 

 the main chain of hills towards the water courses, forming ridges 

 more or less elevated. 



The main ridge is cut down at the heads of the streams into 

 depressions called gaps or passes; the more elevated points are called 

 peaks. The water which has fallen upon these peaks is the origin 

 of the streams which have hollowed out the valleys. Furthermore, 

 the ground falls in every direction towards the natural water courses, 

 forming ridges more or less elevated running between them and 

 separating from each other the districts drained by the streams. 



The natural water courses mark not only the lowest lines, but 

 the lines of the greatest longitudinal slope in the valleys through which 

 they flow. 



The direction and position of the principal streams give also 

 the direction and approximate position of the high ground or ridges 

 which lie between them. 



The positions of the tributaries to the larger stream generally 

 indicate the points of greatest depression in the summits of the ridges, 



