476 



JOHN LYON RICH 



Dominant lateral cutting. — The well-known tendency of a 

 stream to swing from side to side of its valley results in very unequal 

 wear on its channel — active under-cutting on the outsides of the 

 bends being accompanied by deposition on the insides. 



An inspection of Fig. 5 will make clear that lateral cutting 

 alone, a result of the force due to inertia, should result in 

 symmetrical enlargement of the meanders 

 without their displacement. The resulting 

 topographic feature would be the sym- 

 metrical undercut bluff or meander scaur 

 combined, if the stream were cutting 

 down meanwhile, with symmetrical slip- 

 off slopes. A thalweg, therefore, shaped 

 by dominant lateral cutting should pre- 

 sent a pronounced scalloped outline — the 

 greater the dominance of lateral cutting 

 the more symmetrical and the more nearly 

 circular the scallops. That we do not 

 find these symmetrical forms in normal 

 valleys is sufficient evidence that with 

 lateral cutting there is combined another 

 process which destroys the symmetry. 

 That process is siveep. 



Dominant sweep. — ^The down- valley mi- 

 gration, or sweep, of meanders brings the 

 stream successively over all parts of its 

 valley fiat. Any obstructions in the val- 

 ley, opposing the orderly march of the 

 meanders across and down the valley, is 

 attacked. Thus the whole tendency of 

 sweep is toward a clearing-out of the valley 

 by the removal of spurs or other obstructions. It follows, there- 

 fore that, where sweep dominates, only the open type of valley (type 

 I above) is "stable." In the case of a valley of the meandering 

 type, the tendency of the meanders to sweep down- valley leads to 

 more active erosion on the down- valley sides of the meander bends 

 than on the up-valley sides. This results in asymmetry of both 



Fig. 4. — Contour sketch 

 showing typical features of 

 an in-grown meander val- 

 ley, and the effect of sweep 

 in producing assymmetry. 

 Note the unsymmetrical 

 necks of land on the insides 

 of the loops; the sharp 

 undercut bluffs on the out- 

 side and down-valley sides 

 of the bends, and the gently 

 incUned slip-off slopes op- 

 posite. 



