D RA IN A GE MODIFTCA TIONS 667 



(l) RECENT MOVEMENTS INDICATED HY THE MUSCLE SHOALS IN 

 THE TENNESSEE RIVER. 



The Tennessee River is obstructed, in its course throuo-h 

 northern Alabama, by a barrier which is widely known as the 

 Muscle Shoals, and which has been, until the completion of the 

 canal in- recent years, a serious bar to the navigation of the 

 stream. In this case there is no possibility of the existence of 

 an old, abandoned channel around the obstruction, such as 

 characterizes the falls of the Ohio River at Louisville, Kentucky. 

 The Tennessee River at the Muscle Shoals is occupying the same 

 channel that it did in late Tertiary time, hence the obstruction 

 in the stream must be accounted for b}' some hypothesis which 

 admits of the occupancy of the present channel for an indefinitely 

 long time. 



Such a barrier can be produced in one of two v/ays : either 

 the entire region has suffered an uplift and the river has only 

 succeeded in cutting its channel back to Florence, Alabama, or 

 a local uplift has occurred at this point which has elevated a 

 small portion of the stream above its normal grade, and this 

 uplift has been so recent that the stream has not yet been able 

 to remove the barrier. In order to determine which hypothesis 

 best accounts for the facts, it will be necessary to examine closely 

 all of the characteristics of the stream both above and below 

 the barrier. 



{a) Profile of the Tennessee River. — From Chattanooga, Tenne- 

 essee, to Brown's Ferry, Alabama, a few miles below Decatur, 

 there is a fall of but 49 feet in 185 miles ; below Brown's Ferry 

 for a distance of 57 miles the descent is rapid, amounting to a 

 total of 169 feet; below this the grade is again slight, having a 

 fall of only 120 feet in 25O miles. 



From these figures it is apparent that there is not only a 

 break in the grade, but also that the elevation of the head of 

 the shoals is above the average 'grade of the river from Chatta- 

 nooga to its mouth. This evidence appears to favor local uplift, 

 but it is not conclusive. 



