STREAM VALLEYS AND THEIR MEANING 487 



tion of the valley form is very evident. Later stages, where the 

 trenching is deeper but where the down-cutting is still active, are 

 shown by such streams as the Kentucky and the Dix (Fig. 2). A 

 still later stage, when grade has been reached and the formation of 

 a fiat valley-bottom has begun, is excellently shown on the Ver- 

 sailles (Mo.) sheet of the Geological Survey (Fig. 7). A still 

 more advanced stage in a stream which seems to have had a history 

 somewhat like that just outlined is shown by the Black River on the 

 Oberhn (Ohio) sheet (Fig. 8). At the beginning, the stream, in 

 its lower reaches, seems to have had a distinctly meandering course 

 in which it cut down quickly to grade. It is at present changing the 

 form of its meander bends and opening out its valley on account 

 of a relatively rapid down- valley sweep. At the town of Elyria, 

 where the stream has evidently encountered rock of considerable 

 resistance, it is still cutting down and holds its original meandering 

 course only shghtly modified. The valley here is a typical in- 

 trenched meander in its youthful stage of development. Further 

 down, in the softer rocks, a more advanced stage has been reached. 



Slow uplift of a straight stream. — -The results of another type of 

 upHft may be deduced by assuming, as before, an ideal drainage 

 system with the stream flowing in broad, open curves, to be sub- 

 jected to an uplift so slow and continuous that the stream, while 

 forced to continued down-cutting, is, nevertheless, able to maintain 

 itself continuously near grade. 



Under such conditions lateral cutting will assume an important 

 role from the very first, and the stream, though its original course 

 may have been comparatively straight, will come to swing in ever- 

 broadening curves. Continued slow lowering of the stream bed will 

 insure the retention of all width of meander gained by lateral 

 cutting. Sweep will, for reasons already set forth — namely low 

 gradient and continuous cutting in rock — take a subordinate place 

 and will not hinder the further development of the meanders in 

 the bed-rock. As the process continues the stream will sink its 

 bed deeper and deeper into the rock; the meanders will become 

 wider and wider, and the form of the resulting valley will be char- 

 acteristic — the outsides of the meander bends marked by steep 

 undercut bluffs and the insides by more or less gently inclined 



