PHENOMENA AFFECTING RIVER THEORY 225 



races or the water-plane, but he clearly states that the water-plane lies 

 today at 400 feet above tide on the north boundary of the State and at 

 288 feet on the south boundary. These figures are adopted for the datum 

 plane of the Connecticut Valley sealevel waters and are found to coincide 

 very closely with the highest shoreline features. (See the diagram, 

 plate 10.) 



The River Theory; Difficulties and Inconsistencies 

 the upper and the lower valley 



The mouth of the Passumpsic River, at the foot of the '*Fif teen-miles 

 Falls," is made by TJpham the division between the "upper" and the 

 "lower" valley. He found very conspicuous differences between the two 

 sections of the great valley, and these differences are suggestive and im- 

 portant in the light of present knowledge. Speaking of the 20 miles of 

 upper valley, with steep gradient, immediately above the division point, 

 he says : 



"The noticeable features of the valley in this distance are that it is deep 

 and narrow, with sloping sides of till, and destitute of the level alluvial ter- 

 races and intervals which occupy a large width everywhere else along the 

 river" (page 24). 



In contrast, he writes of the lower valley : 



"The modified drift of this lower valley is everywhere well developed and 

 occurs in extensive terraces of various heights, three or four often on each 

 side, the upper one being usually from 150 to 200 feet above the river" . . . 

 (page 26). 



No explanation is offered for the change in the character and amount 

 of deposits at this point in the valley. If the terraces were all river work, 

 then the valley clear to its head should have been filled with detritus, the 

 same as the lower valley, since the source of material was chiefly from the 

 melting ice and filled the valley from south to north, following the re- 

 ceding ice-front. 



The true explanation of the difference between the two sections of the 

 valley is found in the fact that the lower section of the valley was occu- 

 pied by the sealevel static waters, while the upper section was above the 

 marine plane and held only the river. The lower valley contains all the 

 detritus that was swept out of the upper valley by the river flow. By 

 reference to the diagram it will be seen that the hypothetic or datum 

 plane (plate 10) lies at 650 feet at the mouth of the Passumpsic River, 



