84 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



latest glacial filling to have its valley bottom or rock floor reduced to a 

 somewhat regular and rather low gradient. 



EFFECT OF KOCK RESISTANCE ON SIZE OP VALLEY. 



The valuable resistance off'ered by the rock formations has produced 

 an appreciable effect on the size of the valley. It ranges in width from less 

 than a mile up to 5 or 6 miles, and in depth from scarcely 100 feet up to 

 about 800 feet. It is narrow in the deep portions as well as in its passage 

 across hard or resistant rocks, and broad in the shallow portions, which are 

 generally excavated in shales or other soft or easily disintegi'ated rocks. 

 Where the rocks are very easily disintegrated the uplands are greatly 

 broken down for some distance back from the immediate bluffs of the 

 stream, thus giving the valley the appearance of being smaller than it really 

 is; but a full restoration of the original surface in these soft formations 

 would require decidedly more filling than in the hard formations. The 

 effect of rock resistance is discussed in more detail below in connection with 

 the several valleys or valley factors which were antecedent to the present 

 Ohio. 



ROCK ISLANDS IN THE VALLEY. 



In addition to the rock ledges which the present stream encounters 

 there are several noteworthy rock islands in the valley which promise to 

 throw considerable light upon the drainage history. One at the head of 

 the Ohio, in Allegheny, Pa., called Monument Hill, reaches a height of 

 nearly 200 feet above the river, and is separated from the north bluff by a 

 channel less than one-half mile in width, in which there is a gravel filling 

 extending below the level of the present stream. Monument Hill appears 

 to be a remnant of an old gradation plain, and the river there has simply 

 cut a double channel in its olcl bottom. 



Passing over " McKees Rocks," which stand on the south side of the 

 stream about 6 miles below Pittsburg, and seem once to have been con- 

 nected with the north bluff, the next prominent rock island is opposite 

 Steul^nville, Ohio. This island reaches a height of 450 feet above the 

 river, but is crossed by an old gradation plain of Harmons Creek, an eastern 

 tributary of the Ohio, which stands about 350 feet above the river. The 

 present stream occupies only the channel on the west side of the island. 

 The length of the island is about 2 nailes, and the width is scarcely half a 



