QUATERNARY AGE. — RECENT PERIOD. 



559 



In Fig. 944, a section of a valley filled with the Champlain deposits, 

 and thus terraced in consequence of the change of level, is repre- 

 sented. 



In the above explanation, the terraces are supposed to correspond 

 each to a separate period of elevation. This may be the case ; and, 

 when so, the same terrace is traceable for great distances along the 

 course of the larger rivers. But successive terraces may be formed 

 in river-valleys, either (1) during a single slowly-progressing eleva- 

 tion, or (2) in the course of the wear which may be going forward in 

 consequence of a single abrupt elevation ; and it is often difficult to 

 distinguish these accidental or intermediate plains from those that are 

 records of distinct changes of level. One such intermediate terrace 



Fig. 944. 



A 



is shown at r,in Fig. 944. Some of the conditions producing them 

 are the following: (1) changes in the river-channel to one side or 

 the other of the river-valley, altering thereby the action of the flood- 

 waters during freshets, and causing them to commence wear according 

 to a new outline ; (2) resistance to wear in a portion of the alluvium, 

 owing to a degree of consolidation, or to rocks, or some other obstacle ; 

 (3) a permanent diminution in the waters of a stream, arising from 

 changes about its sources, or in some other way. 



Fig. 945. 



33 

















181 



^J7 





















\S7 AC 



53 



\_J8^ 





B 



28 





HI / 

 54 / 













-^K^_2J_/ 



Section of the terraced valley of the Connecticut, at Hadley ; B, a brook; M, Mill River; 

 H, Hatfield ; C, Connecticut River ; H, Hadley. 



Figure 945 represents terraces on the Connecticut, as figured by 

 Hitchcock, and illustrates both the regularities and irregularities of 

 level among them. It is from the vicinity of Hadley, Mass. 



It is important to observe also that the same terrace may differ in 

 height, ten to fifteen feet or more, — because (1) the flood-plains of 



