OBIGJN OF TOPOGRAPHIC POEMS. 117 



confounded with passes, or low points in mountain ranges, caused by the 

 eating away of divides at the heads of streams. 



The valley of every stream above base level slopes not only toward 

 the stream, but with it — i. e., toward its mouth. Every branch on entering 

 the valley feels the influence of this slope and turns its course in greater 

 or less degree down the valley, entering the main stream at an acute angle. 

 Similarly the main stream feels the influence of the tributary and turns toward 

 it; hence the tributary commonly joins the main stream at the head of a bend 

 in the latter. 



When, however, a stream lias recently, by the extension of its drain- 

 age basin, tapped an adjacent stream, the stream so tapped may not yet 

 have accommodated its course to that of the principal stream, so that it still 

 enters it at an obtuse angle. 



Again, when the stream is near base level a different condition is pre- 

 sented. The main stream is on a ridge of its own construction, and the 

 tributary often comes into the valley at a lower level than the ridge and 

 flows parallel with it for a distance before breaking through and joining its 

 waters. Loup fork of the Platte river, Nebraska, is an example of this. 

 The Platte Hows there upon a ridge of its own creation. The Loup comes 

 down into its valley and flows parallel to it for main- miles. 



As was stated before, a stream near base level becomes crooked and 

 winding It has ceased to corrade its bottom, but corrades the sides of its 

 bed, especially at the heads of its bends, and deposits its load on the inside 

 of its bends. Its course changes frequently, now extending its bends 

 farther into the bank and now cutting them off. In this way it eventually 

 excavates a broad alluvial bottom, which may be subject to overflow when 

 the stream is in flood and through which the stream winds in long curves, 

 of size roughly proportional to the magnitude of the stream. 



In the preceding pages no reference has been made to the influence of 

 structure upon topographic forms. The alternation of hard and soft beds 

 of rock and the dip of these beds have decided influence upon topographic 

 forms, which are now to be considered. The influence of these factors upon 

 topography is, it must be premised, greater in the arid regions of the West 

 than in the moister East. The reason of this is that disintegration is much 



