22 PLANT LIFE OF ALABAMA. 



valley is traversed by the Little Mountain range, which owes its exist- 

 ence to the Subcarboniferous sandstones which have resisted erosion. 

 The depression between the main range (Sand Mountain) and Little 

 Mountain widens by their divergence into a broad valley of the same 

 character as the main valley. 



Where the uppermost cherty strata of the Subcarboniferous lime- 

 stone form the surface rock the surface soil is light and more or loss 

 siliceous. This kind of soil pro vails chiefly toward the northern limit 

 of the valley. The soil of the river plain proper, resting upon the 

 lower and more purely calcareous strata, is of great fertility. 



RIVER SYSTEM. 



Its extensive river system forms a most prominent feature in the 

 physical geography of the State. Many of the larger tributaries of 

 the main channels of drainage are navigable, and their innumerable 

 feeders, the creeks and streams, traverse highlands and plains in all 

 directions, atfording an abundant water supply in every part of the 

 State. 



TENNESSEE KIVEK. 



Among the principal rivers is the Tennessee, most remarkable 

 for its anomalous course. It rises in the mountains a short distance 

 beyond the border of southwestern Virginia, and following mainly 

 a southwestern course it becomes navigable at Knoxville. A short 

 distance south of Chattanooga it cuts its channel through a southwest- 

 ern range of the Cumberland Mountains, and enters Alabama at the 

 northeastern corner at Bridgeport. From this point it flows in a 

 nearly uniform southwestern course about 75 miles, when it reaches 

 Guntersville. Here it turns abruptly to the northwest, reaching the 

 Mussel Shoals, 10 miles below Decatur. These shoals are formed of 

 hard, flinty rocks, over which the water rushes in a series of shallow 

 cascades for a distance of about 38 miles, forming an insurmountable 

 obstacle to navigation as far as Florence. From Florence the river is 

 again navigable. It takes a northerly turn at Waterloo, and leaving 

 the State near its northwest corner, continues in this direction, and 

 after a course of 296 miles empties into the Ohio River at Paducah, 

 Ky., making the distance from its source 1,037 miles. 



The vast area south of the Tennessee River is in the main drained 

 by the Tombigbee River and its tributaries, the larger being the Sip- 

 sey and Black Warrior; and the Alabama River with its tributaries, 

 of which the chief are the Coosa and the Tallapoosa. These main 

 channels of drainage h'nd their outlet into the bay of Mobile through 

 the Mobile River, which is formed by their confluence about 50 miles 

 above the river delta. 



