THE WORK OF STREAMS 



IOI 



ancient river valleys which 

 have been widened and 

 deepened by the ice (p. 

 129). 



The Direction of Val- 

 leys. — The direction of 

 stream valleys depends 

 upon a number of condi- 

 tions, some of which can 

 be illustrated by a hypo- 

 thetical case. If a portion 

 of the bottom of a shallow 

 sea is raised above sea 

 level, the land, under these 

 conditions, will have no 

 established stream valleys. 

 When, then, the rain falls 

 first upon such new land, 

 it will gather in places 

 where there are depres- 

 sions and form large or 

 small lakes. Elsewhere, 

 rivulets will flow down the 

 slope, joining here and 

 there in their descent until 

 a stream of considerable 

 length develops. Streams 

 of this sort, whose position 

 and direction are deter- 

 mined by the slope of the 

 original land surface, are 

 called (.1) consequent (Fig. 

 82 A), since their direction 

 is a consequence of the 

 topography of the country, 

 without regard to the char- 

 acter of the rock through 

 which they pass. The 

 streams on the Atlantic 

 Coastal Plain are chiefly 



Fig. 83. — Diagram A shows a region in which 

 a stream flows at grade. Diagram B shows the 

 same region after it has been slowly upwarped 

 athwart the course of the stream. The river is 

 shown as having been able to deepen its valley as 

 rapidly as the elevation occurred. A stream with 

 such a history is an antecedent stream, since it 

 was able to maintain the course it had prior 

 (antecedent) to the deformation of the surface. 



