PHENOMENA AFFECTING RIVER THEORY 227 



relative lowering of the waters, with the reduction in the dimensions of 

 the valley, the southward current became more effective. Consequently 

 the inferior terraces, specially in narrow sections of the valley, undoubt- 

 edly exhibit some effects of the southward currents. It will be difficult 

 to distinguish the lower terraces from the true river plains. It is pos- 

 sible that in narrow stretches of the New Hampshire section some of the 

 delta terraces will show effects of current in the open valley. 



The form and structure of some deltas prove that they were never 

 eroded by any current past their front, though they are bisected and 

 eroded by the stream that built them. The convex front of these deltas, 

 with their constructional frontal slope intact, as shown by the forest beds, 

 prove their construction in quiet water. Mink Brook delta, at Hanover, 

 New Hampshire, shows clearly its constructional frontal slope and lack 

 of undercutting. 



In the wider valley, in Massachusetts and Connecticut, many of the 

 broad detrital plains lie in secluded areas or so sheltered that river work 

 is positively ruled out. a 



TERRACES 



Occurrence. — The benching or terracing of the detrital deposits in the 

 Connecticut Valley, and specially in the narrower New Hampshire sec- 

 tion, is the feature which has been chiefly discussed by the former 

 writers and which has been taken as the main evidence of river work. 



The higher terraces are developed on the deltas, the detrital deposits 

 at the mouths of the side valleys, and they are very weak or entirely lack- 

 ing in the longer intervals between the deltas. This fact discredits the 

 river theory, for a river of great volume and steep gradient, such as must 

 be assumed here, would sweep the detritus down its course and build it 

 as floodplains in the embayments or slack-water areas. 



Discordance. — The want of uniformity in the altitude of the higher of 

 the '^norma?^ terraces and the lack of continuity throws another doubt on 

 their origin as river plains. An example of this discordance may be 

 quoted from IJpham: 



"In Thetford and Lyme we come to an abrupt change in the height of the 

 upper terrace plain. ... At North Thetford this line of the highest terrace 

 suddenly rises to 525 (from 440) and in a mile and a half farther south to 524 

 feet'* (page 35). 



This variation in altitude in short distances, with discontinuity, is a 

 character that does not belong to river plains. Eivers flowing freely 

 make floodplains uniform in gradient and consequent altitude, even if 

 somewhat discontinuous. 



