ee ee ee ee Se Se eee 
3 
Comparison of Evosion and Deposition. 111 
matter contained. The sediments brought down by a Tertiary 
stream, corresponding in location to the present Alabama river, 
were spread over the adjacent sea bottom, mingling on the east 
with the sediments brought down by the Chattahoochee and on 
the west with those brought down by the Tombigbee. It is 
probable that more sediment was brought down by the Alabama 
than by the streams on either side, since it occupies the axis of 
uplift where the greatest erosion took place. Hence if a line be 
drawn midway between the Alabama and Chattahooche on the 
east and between the Alabama and Tombigbee on the west the 
area included would certainly not be wider than the deposition 
area of the axial river. The area included by these lines and 
by the limits of the Ripley and White limestone formations is 
about 6,500 square miles. The thickness of the sediments in 
this area, down to the bottom of the Ripley, varies from 0 at 
the northern edge to 1,900 feet at the southern edge, and their 
volume is about 1,170 cubic miles; but these formations extend 
under the covering of later, deposits, thinning out seaward, and 
while it is impossible to determine their extent or thickness in 
that direction, it seems a conservative estimate to regard the 
volume of the sediments in the seaward extension of the forma- 
tions as equal to that of the actual outcrops. This estimate 
would make the volume of the sediments which may be at- 
tributed to the stream whose lower course occupied the present 
position of -Alabama river during the Tertiary cycle about 
2,340 cubie miles. 
Turning now to the volume of material eroded from the Cre- 
taceous peneplain during the Tertiary cycle by the Alabama 
and its tributaries, the basis for an estimate is somewhat better 
than in the case of the sediments. The greater part of the ero- 
sion has been in the valley of the Coosa and comparatively 
little in that of the Tallapoosa—first, because the vertical dis- 
tance between the baseleyels is greater in the former than the 
latter river basin, and, second; because the rocks are softer and 
hence have been more perfectly reduced. Throughout most of 
the Coosa basin the two peneplains are sufficiently well pre- 
served so that a definite estimate can be made of the material 
removed during the Tertiary cycle. The amount of .elevation 
and distortion which the Cretaceous peneplain suffered at the 
close of the Cretaceous cycle may be determined from a compar- 
ison of plates 5 and 6. It varies from 900 or 1,000 feet at the 
16—Nar. Grog. Maa., vor. VI, 1894. 
