192 H. L. FAIRCIIILD POST-GLACIAL UPLIFT OF N. E. AMERICA 



wide valleys where waves were competent to distribute detritus^ or in con- 

 stricted valleys where the volume of the river was sufficient to produce 

 some current in the estuary^ the plains could not form at the water sur- 

 face. The phenomena of the initial or summit level may be miles away, 

 far inland, and much higher in altitude. 



When extensive delta sand-plains are used as criteria for summit levels 

 they must be judged by the local or neighboring conditions and no special 

 rule or quantitative expression can be given. Many factors are invohod: 

 the volume and velocity of the contributing stream; the volume and 

 character of the detrital load, and the capacity of the water body, with 

 the topography of the under-water slope. 



The small deltas built by little streams, even wet- weather runs, in quiet 

 or shut-in waters are much better indexes of the precise level of the stand- 

 ing water than the heavy deltas of large streams or the broad plains in 

 the open valleys. Of course, these small deltas require sheltered locations, 

 where waves and shore currents are incompetent to remove the delta stuff. 

 The tiny benches, perhaps only rods or yards in area, on the border of 

 small ravines or gullies give the true summit level of secluded waters: 

 and even on the walls of exposed valleys the small-stream deltas may be 

 the best criteria of the primitive water surface. Such small features have 

 commonly been ignored, because they were small and inconspicuous, de- 

 tached, usually inland, and not evidently related to the obtrusive features 

 of the great valleys. 



The deltas of the higher levels in the lateral or inland valleys have 

 been attributed to glacial waters, local ice-border lakes, or pondings due 

 to ice-lobes, and consequently have been slighted. The fact that under 

 certain conditions glacial lakes faced the receding ice-margin certainly 

 requires that caution be used in the study of the water levels, especially 

 in north-sloping valleys. In this study the main reliance is placed on 

 streams with southward flow, or having such direction and relation to 

 the large topography as to avoid blockade by any lobation of the ice-front. 



Deltas and channels of glacial drainage are most positive records of 

 the initial water level. The ice-border streams were ephemeral and they 

 dropped their detritus in the waters that laved the ice-margin, which were 

 then at maximum altitude relative to the land. Sand-plains which can 

 be correlated with extinct glacial streams that debouched in marine estu- 

 aries must give the primitive water level. In the Hudson and Champlain 

 valleys the glacial stream channels and deltas have afforded excellent, 

 precise data. 



