Mississippi Drainage System. — Westgate. 251 
and Arkansas, —a continuation of the line of the present Ohio, — 
and it would seem as if the post-Carboniferous drainage of the 
interior would have discharged along this line. 
Positive evidence for such a discharge is not wanting. Folded 
Paleozoic rocks occur in the Novaculite region of central western 
Arkansas* under conditions which pretty definitely show them to 
be a continuation of the Appalachian folding to the east. Siluri- 
an and Carboniferous rocks are embraced in the folding and the 
structure is closely similar to that of the folded Paleozoic rocks of 
the Appalachian region. The folds are closel}' appressed on the 
south side and flattened on the north side of the area, showing a 
pressure acting from the south. The prevalence of shales on the 
north side and of sandstones on the south side of the region indi- 
cates a derivation of material from the south. In these last two 
respects the Arkansas region resembles the Appalachian. The 
general trend of the folding in the Novaculite area is toward the 
east, — toward the point where the Appalachian folds disappear be- 
neath the gulf Cretaceous. These facts clearl}' indicate a west- 
ward continuation, into Arkansas of the Appalachian folding and 
elevation. In post-Carboniferous time a mountain barrier in all 
probability extended from New York to Arkansas, precluding the 
possibility of a southward discharge of the interior drainage by its 
present outlet, and compelling its discharge to the west, thi-ough 
Missouri and Arkansas. This may be taken as the probable direc- 
tion of the main drainage line of the interior basin in post-Carbon- 
iferous time. 
The change to the present southward drainage seems to have 
taken place at the end of the Cretaceous period. B3' that time 
the Appalachian mountains had been reduced nearly to base-level 
and probably existed as a low and nearl}- obliterated divide. 
When the elevation of the Cretaceous base-level occurred, the 
streams would have followed any troughs produced by the 
warping of the old lowland. The elevation of the Appalachians 
on the east and the plains on the west resulted in producing a 
broad north and south depression in the present Mississippi valle}' 
and in reversing the courses of those streams which had been flow- 
ing westward into the Mesozoic sea. But, perhaps, more impor- 
tant than the elevation of the plains in .diverting the interior 
*Geol. Survey of Ark., Ann. Rept., 1890, vol. iii. Novacnh'tes, by L. 
S. Griswold, pp. 212-214. 
