298 bulletin: museum of comparative zoology. 



The other theories of terracing, regarded as unsuccessful in the pre- 

 liminary inquiry of Part II., should be more thoroughly considered before 

 they are discarded ; but, inasmuch as the further they are examined 

 the less competent they seem to explain the facts observable in the ter- 

 raced valleys of New England, it does not seem worth while to give 

 them any more explicit consideration here. 



Behavior of a Wandering River. The diagrams introduced in 

 the following sections represent several successive stages in the process 

 of slow degradation by a wandering river. The postulates as to river 

 behavior on which the diagrams are based are: (1) The degrading 

 stream continually maintains an essentially graded condition. (2) 

 The lateral swinging of the meandering channel is very much faster 

 (a hundred-fold, for example) than the degradation of the valley floor. 

 (3) The breadth over which a free river (not constrained by ledges) 

 tends to swing laterally is greater than the breadth of the meander belt 

 (the belt included by tangents to the meandering channel). (4) An 

 individual meander tends to enlarge its radius and to work its way 

 down the valley until it may be abandoned at season of high water for a 

 short cut across a flood-plain lobe, or at any season (but usually at high 

 water) for a cut-off through the narrowing neck of a lobe. It is believed 

 that abundant justification may be found for all these postulates, either 

 in the observed behavior of a graded river, or in the success with which 

 they lead to an understanding of the peculiar patterns of our terraces. 

 The several postulates may now be reviewed. 



(1) If the change in the ratio of load to carrying power (volume and 

 slope) proceed very slowly, a river may remain in an essentially graded 

 condition all through the process of aggrading or of degrading its valley, 

 and through the change from aggrading to degrading. It is true that 

 the graded condition depends on a balance between load and carrying 

 power, and it would at first sight appear that any change in either 

 quantity would destroy the balance and throw the river out of grade. But 

 if the change is only by a quantity of the second order — that is, if 

 either load, volume, or slope is changed only by a differential of its 

 value in a unit of time — adjustment to the new condition will follow 

 so immediately that no failure of adjustment will be noticeable. A 

 similar maintenance of an essentially graded condition obtains in the 

 degradation of graded (waste-covered) hillsides as they pass from ma- 

 turity to old age. It is, however, not likely that the very slow degrada- 

 tion of a valley floor which accompanies the advance from maturity to 

 old age in a normal cycle of rock erosion will result in terraces, because 



