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 
