7IO 



ROLLIN D. SALISBURY 



unconformity. This case is less common than the other, and, on 

 the whole, less striking, since lesser age above and greater age below 

 is the rule. In the normal case, however, there is no distinct line or 

 zone of separation. 



Another phase of topographic unconformity will be referred to 

 under "Superimposed Youth." 



II. TOPOGRAPHIC ADJUSTMENT.^ 



The term "adjustment," as applied to streams, has been long 

 in use. A stream is said to be adjusted when it flows as little as 

 possible on resistant formations, and as much as possible on weak 

 ones. In general, adjusted streams follow the strike of the forma- 

 tions over which they run, and where they depart from it they are 

 likely to do so by flowing with the dip for short distances. It is 

 proposed to designate the above type of adjustment structural adjust- 

 ment. Structural adjustment has to do with the courses of streams. 



There is another type of adjustment, namely, topographic adjust- 

 ment, which has to do with the profiles of streams. One or two 

 illustrations will suffice to make the meaning of the term clear. 



I. When a stream has a wide flood-plain, its channel is likely 

 to be now against one bluff, now against the other. This is true of 

 different parts of the valley at the same time, and of the same part 

 at different times. When a stream flows against the bluff on one 

 side for a considerable period of time, the tributaries adjust themselves 

 topographically to this position of their main; that is, the lower end 

 of each tributary has the same elevation as its main at the point of 

 union, and the grade of the tributary above the junction is the grade 

 normal to the stream. If now the main stream shifts to the opposite 

 side of its valley, the tributary stream is thrown out of topographic 

 adjustment. Its lower end is too low, for, essentially without grade, 

 it must find its way across the wide flood-plain to the channel on the 

 other side. The result is that the tributary aggrades its lower course 

 until it has a proper gradient out to its main. When this is accom- 

 plished, topographic adjustment is established. 



If now the main stream swings back again to the bluff which it 

 temporarily abandoned, the tributary finds itself again out of topo- 



I This term has already been pubHshed: Chamberlin and Salisbury, Geology: 

 Processes and Their Results. 



