Davis — Tey^races of the Wesi field Biver^ Mass. 85 



Las been clean swept bv a swingino- curve of the attacking 

 river. This may be explained as a result of the normal progress 

 of a river meander down the valley, until it is stopped bv 

 coming on a ledge, or abandoned by withdrawal of the current 

 to a short-cut or cut-off course. The fact of the down-valley 

 progress of a meander does not seem to have received much 

 attention from physiographers, judging by the sileoce of text- 

 books concerning it ; but it must be a familiar matter to river 

 engineers, so conspicuously is it exhibited on such maps as those 

 prepared by the Mississippi Eiver Commission. The cause of 

 the down-valley progress is evidently to be found in the con- 

 tinued displacement of the thread of fastest current to the 

 down-valley side of the channel on entering the tangent of 

 inflexion between two meander curves. 



9. Broion^s sjnir. — On the other hand, a terrace may trail 

 some distance down-valley from its defending ledge, unless the 

 stream should by any chance swing in again and sweep it 

 away. This chance has not happened in Prospect hill, the 

 spur thus far considered ; but it has in Brown's spur, F. half 

 a mile farther west. This spur is well defended by a large 

 sandstone ledge, at whose forward-reaching base the river is 

 now flowing in a vain effort to widen its valley. Four terraces 

 in the next up-valley reentrant curve forward to the apex of 

 the spur, and all agree in the most unanimous manner to sweep 

 tangent to the slope of the defending ledge. The ledge is well 

 exposed in the cuts made by the passing road and railroad. 

 The scarp of the next higher terrace, G, is, however, pushed 

 back a quarter of a mile farther north ; evidently because when 

 it was made the river was swinging at a slightly higher level 

 than the summit of the defending ledges in the apex of Brown's 

 spur. It seems undeniable, when one looks at these terraces 

 on the ground, that the river would have pushed back the 

 lower members of the series about as far as the higher member, 

 if its lateral swinging had not been stopped by the ledge. 



The peculiar feature of this spur is, however, the close trim- 

 ming that it has suffered on its down-valley side. The river 

 has at least three times swung northward so near the eastern 

 side of the ledge as to narrow the spur into a sharp point. 

 The normal down-valley progress of the meanders cannot be 

 appealed to as a cause of this close trimming on the down- 

 valley side of the ledge ; some special cause must be looked for 

 to direct the several northward swings of the river over so 

 nearly the same course. It seems probable that some constraint 

 has been exerted on the river further up its channel, whereby 

 it has been repeatedly guided to the down-valley side of the 

 ledge. A possible explanation of this peculiar feature will be 

 suggested on a later page. 



