318 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
pected 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 has 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 Swineinc oF THE Meanper Bett. 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 flood 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 
