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 Subearboniferous lime- 
stone form the surface rock the surface soil is light and more or less 
siliceous. This kind of soil prevails 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, affording an abundant water supply in every part of the 
State. 
TENNESSEE RIVER. 
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 
vascades for a distance of about 38 miles, forming an insurmountable 
obstacle to navigation as faras 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 206 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 find their outlet into the bay of Mobile through 
the Mobile River, which is formed by their confluence about 50 miles 
above the river delta. 
