EROSION AL HISTORY OF DRIFTLESS AREA 39 



of evidences being proof of the first cycle, the second set 

 proving- that there was a second cycle carried to a late 

 stage. The evidence of the beginning of the third and 

 present cycle is found in the fact that the second set of 

 evidences is related to a surface distinctly above the present 

 streams. There are certain terrace-like features between 

 the summit areas and the intermediate plain which suggest 

 an additional cycle between the first and second, but as 

 these benches have been proven to be structural and as 

 there are no other evidences in the set, it is concluded that 

 the mountains are in the third, rather than the fourth cycle. 

 The method of procedure then, in determining the number 

 of distinguishable cycles which have been involved in the 

 erosional history of a region is as follows: (1) Determine 

 how many of the evidences of more than one cycle of ero- 

 sion the topography exhibits; (2) sort these evidences into 

 the proper number of sets: (3) conclude that the total num- 

 ber of distinguishable cycles is the number of sets of evi- 

 dences j^liis one. The degree of certainty with which the 

 number of cycles is determined depends upon the certainty 

 with which the various sets of evidences record the in- 

 dividual cycles. 



THE DETERMINATION OF DIASTROPHIC EVENTS 



Because diastrophism is involved in the formation and 

 renewal of lands, the interpretation of the history of land 

 surfaces includes also the diastrophic history. 



The Number of Movements 



In regions in which the land surfaces were originally 

 formed by diastrophism the number of positive diastrophic 

 movements is the same as the number of cycles of erosion. 

 For instance, the Appalachian mountains were formed first 

 by folding; this is movement No. 1. Movement No. 2 in- 

 terrupted the first erosion cycle and inaugurated the second 

 cycle; movement No. 3 started the third cycle. The rrgion 

 has suffered parts of three erosion cycles and there have 

 been three upward diastrophic movements. 



Some surfaces, such as those formed by glaciation. by 



