Mississippi Drainage System. — Westgate. 253 
outcrop of the Carboniferous conglomerate upon the old base- 
level. Beyond the valle}' of the East Tennessee rises the Cum- 
berland table-land, with an average elevation of 1,900 feet' The 
fact that its surface consists of the bevelled edges of strata of 
different hardness, shows that it is a plain of denudation. It is 
another remnant of the old base-level. It is a direct extension 
southward into Tennessee and Alabama of the plateau of western 
Penns3-lvania, which has been identified by Davis as a part of 
the Cretaceous base-level. The western edge of the Cumberland 
table-land is bounded by an abrupt descent to a second plateau, 
which has an average elevation of 900 feet, and which extends to 
the west, completely encircling the central lowland region of Ten> 
nessee, as far as to the Tennessee river and a short distance be- 
yond. This plain has been termed by Safford* "the highland 
rim of middle Tennessee. " On the west this plateau runs be- 
neath Cretaceous beds. The relative straight line of contact 
between the Cretaceous rocks and the Paleozoic rocks which 
compose the highland rim, and the fact that west of the line of 
contact no Paleozoic rocks are seen projecting through the Cre- 
taceous cover shows that the Cretaceous beds were deposited 
upon a comparatively level surface. This level surface appear^; 
to be a part of the Cretaceous base-level which has already been 
recognized in the Cumberland table-land and among the Carolinii 
mountains. A gradual rise from the western to the eastern part 
of the old base-level would lie expected. Such is not the case. 
The two paits are not continuous, but there is an abrupt clitt- 
separating the Cumberland plateau from the highland rim, of 
which the western edge has been considered a part of the Cre- 
taceous l)ase-level. The most probable explanation of the separtt- 
tion of the two parts is that which attributes it to later erosion. 
The elevation of the Cretaceous base-level at the end of Creta- 
ceous time has permitted Tertiary erosion to cut out at a lower 
level a structural plain upon the hard siliceous sub-Carboniferous 
bed which forms the floor of the highland rim. The siliceous 
sub-Carboniferous rocks were the surface rocks of the Cretaceous 
base-level in western Tennessee, hence the Cretaceous base-level and 
the structural plain are identical there, where the Paleozoic rocks run 
l)eneath the Cretaceous. But in midtlle Tennessee the siliceous 
*For a description of the physical features of Tennessee see Safford, 
Geology of Tennessee, pp. 21-125. 
