318 BULLETIN': MUSEUM OF COMPARATIVE ZOOLOGY. 



pectcd in association with defending ledges ; yet they must all conform 

 to certain general laws of development. 



If the ledge lies at a low level, the greater part of the terraces that 

 have been cut at swings of higher level will have been destroyed before 

 the ledge lias a chance to defend them. If the ledge is of large size, 

 rising nearly to the highest terrace level and standing forward in such a 

 position that the stream may frequently swing against its buried slope, 

 a whole flight of stepping terraces may be not only formed, but pre- 

 served by it. Here the early terraces, unlike those of free-swinging 

 rivers, are defended by ledges, and cannot here be attacked by 

 the later swings and sweeps of the stream. They are subject to destruc- 

 tion only by general weathering and washing of the valley sides. It is 

 evidently, then, to the largest and highest and most outstanding ledges 

 that one must go in order to find the fullest record of the number of 

 swings that a river has executed during the excavation of its valley, for 

 only on such ledges are the records of river terracing well preserved. 

 Elsewhere they are for the most part swept away. Even here some 

 swings may not be recorded. In short, the maximum number of ter- 

 races shows only the minimum number of river swings. 



Diminished Swinging of the Meander Belt. The greater the 

 depth to which the valley floor is degraded, the more frequently may 

 ledges be found, and, as a rule, the nearer will they stand to the axis of 

 the valley. The number of defended cusps will therefore tend to in- 

 crease as the valley deepens. The breadth of free swinging will at the 

 same time decrease, and the space between the scarps of the lower ter- 

 races will necessarily be less than the space between the higher terraces. 

 This principle, first stated by Miller, seems to be essential in explaining 

 the stepping terraces of New England. 



It must frequently happen that ledges approach the axis of a valley 

 more closely at one point than at another. The valley may be well 

 beset with buried reefs for a fraction of a mile or more, and then may 

 be relatively free from ledges for several miles up and down stream. 

 Where the ledges are numerous, the valley will be narrowed, and the 

 terraces will be preserved in good number; but in the stretches that are 

 comparatively free from ledges, or in which ledges are found only at 

 low levels, the valley floor may be broadly opened, and but few of the 

 many Hood plains that the river there formed at various levels will 

 be preserved. These open basins, often bordered by a single high- 

 scarped terrace, have attracted less attention than they deserve in 

 the discussion of terracing ; and well-developed flights of terraces 



