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 Xew 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 



