DAVIS: RIVER TERRACES IN NEW ENGLAND. 291 



of the Connecticut valley in Massachusetts were, according to Emerson, 

 deposited in lakes or bodies of slack water at or very close to the sea- 

 level of their time, but the clays now reach elevations approaching two 

 hundred feet. No postglacial changes of level of such amounts are 

 known to have taken place along the southern New England coast. Our 

 south-flowing rivers have therefore been accelerated, while those flowing 

 northward have been retarded ; and to this differential tilting Shaler 

 has ascribed the weak terracing by streams of the latter class in contrast 

 to the active terracing by those of the former. 



While it is thus made very probable that the erosion of valley drift 

 was determined by the unequal elevation of New England in postglacial 

 time, it does not follow that individual terraces are in any close way 

 related to this movement. Several cases must be here distinguished. 



The northern uplift may have been accomplished in a single movement 

 and so rapidly as to have revived the streams to an unusual activity of 

 erosion, whereby they deepened their valleys quickly for a time, and did 

 not begin to swing laterally, in the manner essential to terracing, until 

 they had developed new grades of gentle declivity after the rapid uplift 

 had ceased. In this case only a single high-level terrace and no inter- 

 mediate terraces would be formed, and there would be but few low-level 

 terraces. 



A second supposition includes cases of repeated rapid uplifts separated 

 by deliberate pauses, each of which would produce a result similar to 

 that of the previous case. Here we should expect the river to have 

 swung laterally at as many different levels as there had been pauses 

 during the total uplift ; and the flood plain formed during each pause 

 would be of relatively persistent occurrence down the valley. But in 

 order to protect the terrace remnants of the successive flood plains from 

 being consumed by the river when it swings from side to side at lower 

 levels, it is necessary to postulate that the movements of uplift should 

 succeed each other at shorter and shorter intervals, so that the later-carved 

 flood plains should be narrower than the earlier ones. The chief objection 

 to this supposition is not so well directed against the postulate just men- 

 tioned as against the requirement of correlated levels in the terrace on 

 the two sides of a valley. Such correlation is occasionally found, but it 

 is by no means characteristic of our terraced valleys in general. The 

 terrace levels are usually so discordant on the opposite sides of a valley 

 that they cannot be considered the records of still-stands of the land 

 between times of rapid uplift. 



A third supposition considers an uplift so slow that the south-flowing 



