132 PRINCIPLES OF STRATIGRAPHY 



II. Inscqnent Streams. These are essentially distinct from the 

 preceding type in that they originate upon the side of a preexisting 

 valley of erosion or dislocation, and in that their chief mode of 

 development is hy headward gnawing instead of downward cutting. 

 Nevertheless, there are times when it might be difficult to dis- 

 tinguish this type from a consequent originating on an erosion 

 surface, or one flowing down the back-slope of a fault block. For 

 the question would arise, when is the erosion slope steep enough 

 to support insequents instead of tributary consequents? In the 

 case of the fault block, we may consider that the streams on the 

 back slope are consequents and those on the fault scarps are inse- 

 quents. Of course this merely emphasizes the fact that in any 

 natural classification of inorganic, as well as of organic things, sharp 

 lines do not exist, but the classes are connected by gradations. 



Most insequent streams are classifiable into a. erosion-blufif and 

 b. fault-scarp insequents. The former comprise ( i ) wave-cut cliffs 

 (marine and lacustrine), (2) river-cut cliffs, (3) ice-cut cliffs, (4) 

 wind-cut cliffs, and (5) artificially-cut cliffs, while the second 

 includes scarps due to faulting and to local subsidences, such as 

 kettle-holes in moraines, etc., sink-holes over caverns and others. 

 A third but rare type c. forms on the margins of pit or explosion 

 craters, while a fourth type d. is seen in embankment insequents, 

 where cliffs of construction or artificial embankments are gullied. 

 This type approaches the consequent type developed on dry river 

 deltas or alluvial fans, since it develops on a constructional surface. 



Simple subsequent streams. Insequent streams, near the head 

 of a coastal plain, may open out a lowland parallel to the strike of 

 the strata on an underlying softer substratum. Such enlarged 

 insequents have been termed subsequents, and in a young region 

 these may be of simple type. In most cases, however, subsequents 

 become compound through capture. Axial subsequents open up 

 longitudinal valleys on the crests or axes of anticlines, and by 

 sliding downward occupy the monoclinal valleys. (See chapter 

 XXI.) 



Obsequent streams. These are insequents peculiar on account 

 of their location on the inface of a cuesta, as a result of which 

 their course is in an opposite direction from that of the consequents. 

 Rivers on the fault face of tilted blocks would also come under this 

 category. 



III. Overfloiv Streams. These are the spillways from stand- 

 ing water bodies and as such include all effluents of lakes. Such 

 effluents may be terminal, carrying the waters of the lake directly 

 to the sea or into a trunk stream which flows into the sea without 



