196 H. L. FAIRCHILD POST-GLACIAL VPLIFT OF N. E. AMERICA 



These aggraded river deposits are at least 60 feet higher than the estu- 

 ary levels a few miles below at Barrett and Sugar Hill, and they extend 

 upstream over a mile to Apthorp, where the plain is about 30 feet higher 

 than at Littleton, being given as 814 feet. 



Below Littleton the valley widens, with broad lateral embayments at 

 the junction of the tributary valleys. In these embayments are high- 

 level deltas, sculptured into terraces, which mark falling stages of the 

 estuary waters. The summit planes at Barrett and Sugar Hill were 

 estimated at 705 to 725 feet. The theoretic altitude, according to the 

 isobases, is about 720 feet for the marine plane. That these terraces on 

 the detached deltas are the work of standing water, combined with the 

 depositional work of the inflowing streams, is perfectly evident. Ter- 

 races are limited to the deltas, with no benching of the till walls of the 

 valley. The width and irregularity of the valley rules out river work. 

 Precisely the same characters belong to the terraces of the Connecticut, 

 Hudson, Champlain, and all the large valleys open to the sea. 



Another point in way of discrimination may be noted. In the narrow 

 river section of the valley there are no deltas at the mouths of the tribu- 

 tary valleys, as the contributed detritus was carried away by the trunk 

 stream current. And the lines along the valley sides have a continuity 

 and straightness impossible to weak wave action. On the other hand, the 

 convex and irregular outlines of the delta fronts in the estuary section 

 of the valley, with absence of erosion lines on the till salients of the valley 

 walls, rule out river work. 



To most students all this will seem very elementary and quite common- 

 place; yet the significance of these features appears not to have been 

 recognized by those who regarded the high-level terraces of wide valleys 

 as the work of enormously flooded rivers. 



IXITIAL WATER PLAXE ON DELTAS 



A heavy delta in a relatively narrow valley may be built far out in and 

 ])eneath the static water and also be aggraded upstream far beyond and 

 much above those waters. On these ancient "fossil" deltas we may have 

 a horizontal extent of several miles and a vertical range of even 100 or 

 200 feet. Such is the character of many of the heavy deltas built by the 

 glacial outwash and later land streams at the initial plane of the sealevel 

 waters. The discrimination of the subaqueous portion of the primitive 

 delta from the subaerial, aggraded part becomes important for the close 

 determination of the earliest marine plane. In other words, the problem 

 is to locate along the sloping surface of the delta the intersection of the 

 initial or highest static water level of the estuarv. A close determination 



