68 PHYSICAL GEOGPAPHY. 



river flows. It is bordered upon either side by more or less 

 abrupt hills or bluffs, which have been left standing while the 

 valley was gradually being deepened by the erosive action of 

 its own waters. The bottom of this valley, between the 

 ranges of bluffs npon each side, varies from one or two to six 

 or eight miles in width. The whole space between the bluffs 

 is occnpied by the river and its bottom or flood-plain only, if 

 we except the occasional terraces or remains of ancient flood- 

 plains which are not now reached by the highest floods of 

 the river, and have never been reached by them since the 

 river attained its present level. The height of the bluffs 

 above the flood-plain varies from one hundred and fifty feet 

 to upward of four hundred feet, the highest being at the 

 northern part of the State, where also the valley is narrowest. 



The Lower Silurian formations compose the bluffs there, but 

 they gradually disappear by a southerly dip, and the bluffs 

 are continued uninterruptedly by being successively composed 

 of the Upper Silurian, Devonian, and sub-Carboniferous rocks. 

 Each of these formations, according to its lithological compo- 

 sition, modifies to some degree the outline and aspect of the 

 bluffs. Thus, from the northeast corner of the State to a 

 point several miles below Dubuque, the valley has a deep, 

 broad, gorge-like appearance compared with what it has 

 below. This is largely due to the firm character of the two 

 great magnesian limestone formations, that compose the 

 greater part of the bluffs along that portion of the valley. 

 Below this, the bluffs are composed of the Niagara, and 

 Devonian limestones and are more retreating and rounded 

 in their outlines, but with diminishing height, they recover 

 something of their abruptness upon the accession of the 

 sub-Carboniferous limestone near the southeastern corner of 

 the State. 



The river itself is from half a mile to nearly a mile in width, 

 and winds from side to side through the flood-plain, so that it 

 runs near the foot of the bluffs upon one side or the other, 

 along a great part of its course. There are, however, only 

 three or four points along the whole length of the State where 



