Mississippi Drainage System. — Westgate. 257 
the whole of eastern North America,. The amount of elevation 
was not the same in different parts of the area. The plateau now 
stands at an elevation of 1,300 feet in western New England, 
1,600 feet in New York on the Hudson, and 2,500 feet in western 
Penns3lvania. These elevations represent in a general way the 
amount of Tertiar}' elevation and warping: for deformations of 
post-Tertiar}' date are of relatively' small amount. Southwestward 
along the Appalachians the altitude of the plateau falls until in 
Georgia and Alabama it dips beneath the gulf Cretaceous and be- 
neath the sea level as well; so that across the lower Mississippi 
there was no elevation, but an actual subsidence. In the upper 
Mississippi valle}^ lack of recognizable remnants of the old plateau 
prevent an accurate determination of the elevation, but the amount 
was less toward the Mississippi and in IMinnesota was probably of 
very little account. 
By the elevation and warping of the Cretaceous base-level im- 
portant changes were effected in the drainage of the eastern Mis- 
sissippi basin. The elevation of the plains turned the drainage of 
a large part of the interior toward the east. All the rivers which 
now cross the plains date only from the earl v Tertiary*, for during 
Cretaceous time the present area of the plains was beneath sea 
level. The Missouri and its tributaries are Tertiary rivers. The 
elevation of the plains on the west and the Appalachian area on 
the east formed a broad north and south depression which deter- 
mined the course of the upper'Mississippi: and, aided by the de- 
pi'ession of the base-level across the present course of the lower 
Mississippi, turned the main drainage discharge of the interior 
southward by its present course to the gulf. The Mississippi, 
therefore, also dates only from the commencement of the Tertiary-. 
These changes in drainage were caused by the warping which 
accompanied the elevation of the old lowland rather than by the 
elevation itself. Upon rivers flowing over approximately horizon- 
tal rocks such as cover most of the Mississippi basin, the only ef- 
fect of elevation alone would be to increase the erosive power of 
the streams; it would not alter their courses. A ditt'erential up- 
lift or warping is necessary to cause a change in the direction of 
drainage courses. 
With the elevation of the Cretaceous base-level the rivers began 
to cut down to a new base-level and entered upon a new cycle of 
development. This is the Tertiary cycle of development. With 
