STREAM VALLEYS AND THEIR MEANING 481 



yields rapidly and the locus of attack on the rock of the valley side 

 is shifted down stream before any great impression on the valley 

 walls can be made. A repetition of this process with its constant 

 down-stream migration of the locus of attack must produce rela- 

 tively straight valley walls. 



In a stream of constant volume, as the gradient gradually 

 decreases with advancing age, the tendency to sweep (dependent 

 on the gradient) decreases, while the tendency toward lateral 

 expansion of the meander loops increases (since the latter becomes 

 relatively more effective as the gradient becomes less) . The stream, 

 therefore, remains against its rock banks at any given place longer 

 than before with the result that the straight valley wall character- 

 istic of its earlier stages gradually gives way to a series of crescen- 

 tic scaurs marking the successive points of impingement — the older 

 ones, perhaps, considerably modified by general weathering since 

 their formation.^ 



In the second case, when a stream has ceased down-cutting 

 altogether or has so nearly ceased that the amount of such cutting 

 is at a minimum, down- valley sweep, combined, of course, with 

 lateral cutting, will take first place and will work out its char- 

 acteristic results whatever may have been the original form of the 

 valley. In valleys of either the intrenched or the in-grown mean- 

 der type the stream will develop a fiat flood plain, first on the 

 up-valley sides of the bends and on the convex banks of the loops; 

 gradually it will cut off any spoon-shaped necks of land between 

 the loops, perhaps producing "rock islands" as a by-product, and 

 finally it will remove all spurs or isolated rock masses.^ 



The third case, streams carrying coarse material, is best illus- 

 trated by the small headwater tributaries of many streams, par- 

 ticularly those in regions of resistant rocks. A plentiful load of 

 coarse material means a high gradient, for only a swift stream is 

 "competent" to the coarse debris.^ The high gradient gives high 

 values to the forces responsible for down-valley sweep while the 



' This feature is well shown on the WilHamstown (N.C.) topographic sheet. 



2 See W. M. Davis, "River Terraces in New England," Bull. Mas. Comp. Zool., 

 XXXVIII (1902), 281-346. 



3 Gilbert, Henry Mountains, chapter on "Land Sculpture." 



