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bulletin: museum of comparative zoology. 



The highest plain of a flight of terraces backs against the ascending 

 slopes of the older valley side and accepts their outline as its border, as 

 in Figure 6 ; while each lower terrace, as well as the existing flood plain 

 — the " intervale " or " interval " of New Englanders — backs against 

 the scarp of the next higher terrace ; thus the intermediate members of 

 a flight of terrace steps possess similar but not necessarily parallel out- 

 lines, front and back ; the cusps between the curves all point towards 

 the stream. The back border of a terrace i3 frequently followed by a 

 marshy channel from which the terracing stream has been withdrawn 

 by a short-cut or cut-off (as is more fully considered below) before the 



Fig. 6. 



channel was filled ; terrace plains thus characterized may slope gently 

 away from the axis of the valley towards their back border, and if they 

 are of moderate breadth the backward slope may be a relatively con- 

 spicuous feature, as in the lower terrace in the middle of Figure 6. 

 Terrace of this kind were called "glacis terraces" by Hitchcock (58). * 

 They are of very common occurrence, and serve to show that the sudden 

 withdrawal of the terracing stream from a roundabout channel to a more 

 direct course has not been unusual. 



The scarp of a terrace connects the front border of the plain above 

 with the back border of the plain below. Its sloping surface therefore 



1 Numbers in parentheses after an author's name are page references to his 

 writings, cited in the Bibliography. 



