314 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
curve is to remain practically unchanged, it can pass the ledge only by 
withdrawing somewhat towards the axis of the valley; it may thus, as 
it were, slip by the obstacle that stands immovable in its way. The 
stream is represented as having just slipped past the Jedge in Figure 27, 
and as having swept somewhat farther down the valley in Figure 28. 
All records of the first and third swing of the river are now destroyed, 
so far as this part of the valley is concerned ; the terrace front shows a 
high, defended, one-sweep cusp, a free two-sweep cusp with an up-stream 
Y-stem, and a free one-sweep cusp. 
The ledge at the base of the defended cusp may come to be more or 
less concealed by the sands that are washed down from the weathering 
scarp. In time it may be entirely covered, and its presence will then 
be known only if a road cut or a boring reveals it. It is therefore quite 
possible that some apparently free cusps are really defended cusps, with 
their defending ledge ambushed beneath a thin cover of soil. 
CoMPRESSED MEANDERS AND SuHarp Cusps. Let it now be supposed 
that the river in Figure 26 is unable to slip past the ledge. The front 
