48o JOHN LYON RICH 



would increase the inertia component without altering either of 

 the other two. 



In respect to the third of the factors determining the rate of 

 sweep, it is of course evident that this rate will, other things being 

 equal, depend directly on the resistance of the material against 

 which the stream is working. If this material is the soft alluvium 

 of a flood plain, the meanders will sweep past rapidly; if it is 

 resistant rock they will go much more slowly. If the banks are 

 low, less material must be moved than if they are high, consequently 

 the meanders will tend to move more rapidly. 



Where the meanders are intrenched, lateral, vertical, and down- 

 valley erosion will be equally affected by the character of the rock, 

 but where a stream is flowing on a flood-plain-floored valley with 

 rock bluffs on the sides, lateral cutting into the valley walls is 

 opposed by rock, while sweep is opposed only by soft flood plain 

 alluvium of moderate thickness. The result must be a relatively 

 rapid down- valley migration of the curves of the river. 



Conditions under which sweep becomes dominant. — ^The ratio 

 between the rate of sweep and that of down-cutting is one of the 

 fundamental' factors in determining the form of a valley. Three 

 common conditions lead to dominance of sweep. These are: 

 (i) a stream at grade in a relatively straight valley; (2) a stream 

 which for any reason, for instance a decrease in gradient, has 

 ceased down-cutting altogether; and (3) a stream carrying coarse 

 material. 



In the first of these cases a stream, having cut down to its pro- 

 file of equilibrium in a relatively straight valley, still continues to 

 deepen this valley, but only slowly, as the region as a whole becomes 

 lower. The stream swings from side to side in ever-broadening, 

 open curves. The tendency to down-valley migration of these 

 curves is opposed only by the resistance of the thin coating of 

 relatively weak flood-plain material on the valley flat, plus that of 

 the thickness of rock which represents the amount of down-cutting 

 in the interval between the passage of successive meanders. The 

 tendency toward lateral cutting is opposed by the rock of the valley 

 walls. The result is that when the stream is turned against the 

 valley side, the flood-plain material on the lower side of the bends 



