288 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



The several theories by which terraces have been explained may 

 now be reviewed. 



Terraces carved by Streams of Diminishing Volume. The primi- 

 tive explanation of terraces is that the whole space between the upper 

 terrace scarps represents the channel of a huge river by which the valley 

 was once drained, and that successive diminutions of volume to that of 

 the present river are indicated by the decrease of breadth between the 

 terraces in descending order. Although this view has sometimes re- 

 ceived distinguished advocacy, it has never gained general acceptance 

 among geologists or geographers. It has, however, been very generally 

 supposed that the present rivers are much smaller than they were 

 when they began the work of terracing ; hence it is desirable to con- 

 sider the special features that should appear if a decrease in stream vol- 

 ume had actually taken place. 



The best indication of the volume of the stream by which a terrace 

 has been carved is afforded by the curvature of its frontal scarp. If the 

 scarps of the low-level terraces have a radius and an arc of curvature 

 similar to these elements in the existing river meanders, and signifi- 

 cantly smaller than in the high-level scarps, while curves at interme- 

 diate levels show intermediate values, a diminution of stream volume 

 may be fairly inferred. If the radius and arc of curvature are of about 

 the same measure in the three cases, no change in stream volume is 

 indicated. 



On the other hand, if a stream were charged with abundant and 

 coarse load in the last stages of its aggrading action, as seems to have 

 been frequently the case in New England, its slope must have been 

 relatively strong ; and a graded river with a heavy load on a strong 

 slope does not develop curves of as small radius as it would when sub- 

 sequently flowiug with the same volume but with a finer load on a gen- 

 tler slope ; hence a large radius of curvature in the uppermost terraces 

 should not alone be taken as an indication of large volume ; large arc of 

 curvature should also be found before large volume is inferred. It is 

 for this reason that some of the uppermost terraces of the Connecticut 

 and the Westfield rivers, whose scarps seem to sweep in curves of 

 greater radius than do those of the low-level terraces, cannot alone give 

 assurance of a former greater volume for their rivers. 



It is, however, rendered very probable by what is known of the later 

 stages of the glacial period that many of our streams had greater volume 

 then than now. The most effective cause for greater volume was the 

 constraint of the ice sheet, whereby the drainage from the basins of 



