6 INTRODUCTION. 



very slight it is true but still perceptible, the hills presenting 

 gentle slopes and smooth, rounded outlines; then the valley 

 begins to narrow, the hills are more sharply outlined, and pres- 

 ently the stream is running between precipitous walls. At in- 

 tervals the valley will expand to a width much greater than is to 

 be found for many miles above or below; and after holding a 

 practically uniform width for some distance will rapidly contract. 



Modifications of this character are usually asserted to be due 

 to the diversified composition of strata through which the river 

 makes its way. To the same cause, too, are assigned the fre- 

 quent abrupt curves, some of them so sharp that the river 

 seems almost to double back on itself. There are, to be sure, 

 many degrees of hardness and of solubility in all the rocks 

 through which the Ohio has cut its channel; and these prop- 

 erties would certainly be factors in the phenomena ob- 

 served. But, even where these features are most pronounced, 

 the rock seems to be tolerably homogeneous in its structure; 

 and it does not seem reasonable to suppose that inequalities 

 of this nature would be so capriciously distributed as would have 

 to be the case were they the only or even the principal cause 

 of such conditions. 



In recent years much thought has been given to these ques- 

 tions, and some investigations conducted mainly by Prof. Tight 

 as shown by his article have given us the key to the problem. 

 It is very easily answered; being simply the fact that prior to 

 the glacial period the Ohio as a separate stream had no exist- 

 ence. Its present channel was occupied by a series of discon- 

 nected water courses, varying in size from small ravines to large 

 rivers. The expansions in its course are the valleys of the larger 

 pre-glacial streams ; the abrupt curves and numerous windings 

 result from the efforts of the stream to find the lowest level in 

 broken and irregularly eroded country across which it must 

 seek a path from one valley to another; and the narrows or 

 gorges mark the places where it broke through the minor water- 

 sheds that obstructed its progress. 



The following pages contain the result of examinations made 

 within the past two years, under the auspices of the Ohio Acad- 

 emy of Science. There are some references in the text that 



