120 



A MANUAL OF TOPOGEAPHIC METHODS. 



Consider next the case of a stream flowing parallel to the strike of 

 inclined beds, where they are alternately hard and soft. When the incli- 

 nation of the beds is not great, the stream, having cut down to the surface 



i* of the hard bed, as represented in 

 No. 1, Fig. 14, tends to travel later- 

 ally down the dip of the bed, under- 

 mining both soft and hard beds on 

 the lower side and extending the slope 

 on the upper side. When the dip is 

 considerable, it may carry away all 

 the material on the upper side, as 

 in No. 2, Fig. 14. 



In this way streams may cut broad 

 swaths across the terrain and remove 

 both hard and soft beds from great 



FIG. U.-Cross sect s i„ taclined beds. ., m|s Q f mc lj ne d plateaus. 



Fine examples of streams flowing on the strike of hard inclined strata 

 are seen in the hogbacks of Colorado. 



Next, consider the longitudinal profile of a stream which is cutting its 

 bed, when flowing over a succession of beds alternately hard and soft. 

 Since it cuts soft rocks more rapidly than hard ones, its profile will show 

 irregularities. Where»flowing over soft beds, its current is less rapid than 

 over hard beds of rock. The. stream adjusts its profile to the work to be 

 performed. 



The ultimate result of aqueous erosion upon a surface is to reduce it 

 to" a plain of slight elevation, of gentle, easy slopes. It then approaches 

 base level, a condition where the entire surface resembles the condition of 

 a base-level stream, where vertical corrasion is practically at an end. Abso- 

 lute base level is a conception merely, which many regions approach, but, 

 owing to the fact that as the slopes become gentler, erosion becomes feebler, 

 they cannot reach. 



The stage of progress of an area toward base level is said to indicate 

 its age. In youth it may present a great elevation and high relief. Its 

 streams may have rapid courses with irregular profiles, broken by lakes, 



