112 Part II. Chapter 1. 
f. The Great Valley. 
The Great Valley between the Appalachian and the Cordilleran mountain 
systems is mainly drained southward to the Gulf of Mexico by the Missis- 
sippi River and its tributaries the Minnesota, Des Moines, Missouri (and 
its affluents), Arkansas and Red river, flowing from the west, besides the 
Illinois, the Ohio (and its branches the Wabash, Cumberland and Tennessee 
rivers) and Yazoo rivers emptying into it from the east. This valley is a great 
level plain, except where scored by streams. In the east, small prairies existed, 
while beyond the Mississippi, the prairies were interrupted only by the groves 
that border the streams. This great prairie sweeps around the Ozark hills 
in Missouri, Arkansas and Oklahoma and extends southward to the Rio Grande 
del Norte. The great ice sheet covered this prairie down to the mouth of 
the Ohio River, and wherever it was spread, beds of clay, sand, gravel and 
glacial hills are found. The prairies merge imperceptibly on the west into the 
region known as the “Great Plains”, but which are really topographically 
speaking great plateaus. Below the Ohio River on either side of the Missis- 
sippi River, occur the Gulf plains named by Powell, the east Gulf plain, the 
west Gulf plain, the flood plain of the Mississippi and coastal marshes and 
the submerged plain, extending into the Gulf. The most interesting of these 
is the Mississippi flood plain, which starts at the foot of the great glacial 
deposits in Illinois and at the Gulf end expands into a great delta built by 
sediments brought down by that great river. The outlines of this flood plain 
are marked by high bluffs of loess. This great plain seems to have been 
somewhat more depressed during glacial time, than at present, and the water 
from the glacier gathered into a great basin which emptied into the Gulf. 
The Ozark Mountains extend southward and westward from the Iron 
Mountains of Missouri. The region north of the Arkansas River has been a 
great plateau, but it is now deeply trenched by numerous winding streams. 
The rocks of this region in the southern district bend in a synclinal fold and 
are eroded into long parallel mountain ridges with intervening valleys. The 
streams in the most southern portion of the region are transverse to the rock 
structure. 
The Great Plains, or plateau region, extends from British America to 
the Rio Grande River. The Missouri district is deeply cut by the Missouri 
River and its tributaries. The Platte plateau is. drained by rivers that flow in 
a general easterly direction into the Mississippi River, while the Arkansas 
plateau is drained by the Arkansas, the Cimarron and two of the Canadian 
rivers. The Pecos tableland in Texas is drained by the Pecos and the tribu- 
taries of the Red, Brazos, Neuces and Colorado rivers, and is known also as 
the Llano Estacado, or Staked Plain. 
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