292 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
rivers were never much accelerated ; for during slow uplift the larger 
rivers might continue to swing actively from side to side, while all the 
time degrading the valley floor. In this case terraces might be cut at 
many different levels on opposite sides of the valley, according to the 
habit of the river in its lateral swinging. 
The third supposition seems most appropriate to New England, for all 
of our valleys in which terraces are well developed exhibit flood-plain 
remnants at many levels, high and low, and neither so few in number 
nor so accordant in relative altitude above the river as to imply that 
lateral swinging had occurred only during the quiet intervals between 
rapid uplifts. But it should be noted that this conclusion applies better 
to the valleys of good-sized streams or rivers than to those of small 
brooks ; for the latter frequently show only faint terraces or no terraces 
at all, even though they are branches of rivers whose valleys are well 
terraced. This seems to mean that an uplift which was so slow that a 
good-sized river could easily keep pace with it by down-cutting, may 
have been too fast for such a result in the case of a small stream. While 
the able-bodied rivers may thus have been always effectively at grade, 
leisurely swinging from side to side and at the same time slowly wearing 
down their valley floors, the small streams may have been for much or 
all of this time above grade, and therefore unable to widen their little 
valleys, although actively engaged in deepening them. On the other 
hand, even the largest rivers have not been able to maintain a graded 
channel in the rock ledges upon which they have been here and there 
superposed by the drift cover. They are still actively cutting down such 
ledges, but they are not yet able to widen the rock-notch that they are 
cutting ; thus imitating the condition of their smallest branches, which 
have not yet been able to widen their little valleys even in clays and 
sands. Boulder clay or till is of a resistance between the feebleness of 
stratified drift and the strength of rock ledges. If a mass of till is 
discovered the stream may be successful in cutting down its channel to 
grade, and yet unsuccessful in opening a valley floor ; and thus a boul- 
der-clay ‘‘shut in” may be produced between open valley floors or 
“intervals” that have been eroded in weak stratified drift farther up 
and down stream. Little river, a mile southwest of Westfield, Mass., 
offers examples of this kind (page 333). 
The small changes made in rock ledges during the development of an 
extended series of river terraces serves to indicate how short is the 
duration of the episode in which the alluvial filling of a valley is terraced, 
in comparison with the time needed for the erosion of the rock-bound 
