TERTIARY WORK IX THE OHIO VALLEY. 575 



this part of the plateau ; owiug to the gentle uplift here, their depth is, 

 however, moderate compared to the great gorges of the Hudson, the Dela- 

 ware, the Potomac, the James and the Staunton, which trench the inner and 

 higher division of the crystalline belt. The continuance of this feature to 

 the southward needs further study. 



In western Pennsylvania and in the parts of Ohio thereto adjoining, the 

 valleys of the plateau are wider opened than near its eastern border. There 

 appear to be several reasons for this change : In the first place, the rocks of 

 the plateau decrease in hardness in passing from east to west — that is, in 

 receding from the ancient coast-line from which their sediments were 

 derived ; in the east they are sandy ; in the west, shaly. In the second 

 place, the streams in the west are larger and nearer their mouths, while near 

 the eastern border of the plateau they are as a rule small head-waters, whose 

 beginning of work had in great measure to wait until some considerable 

 advance in valley-making had been accomplished farther down-stream. In 

 the third place, the height of the Tertiary uplift does not seem to have been 

 so great in Ohio as farther eastward ; but while this may have had some 

 effect on the speedier development of open valleys, it does not appear to be 

 so important as the other two considerations. 



In Kentucky, where there are distinct alternations of hard and soft 

 members of the plateau mass, we find a kind of form that is not present in 

 any significant degree in Pennsylvania or Virginia ; namely, structural 

 plains. By this term I would designate an even surface of a hard stratum 

 from which a certain amount of superincumbent material has been denuded, 

 and on which further denudation hesitates by reason of the great hardness 

 of the bed then discovered. Button's account * of the successive plateau-like 

 steps surrounding the San Rafael swell in Utah presents the best illustration 

 of structural plains on a large scale that I can quote. Those of Kentucky 

 are tame affairs in comparison, but they characterize the moderate relief of 

 the central part of that State. Structural plains are to be distinguished 

 from constructional plains by the signs of denudation by which they are 

 surrounded. They are to be distinguished from baseleveled plains by their 

 accordance with the structures on Avhich they are developed. In a small 

 way, the sloping backs of the Medina ridges in Pennsylvania are inclined or 

 dipping structural plains ; f so are the backs of the lava ridges of New 

 Jersey and Connecticut ; but in neither of these cases does the surface of 

 the ground coincide so closely or over so large an area with the surface of 

 the disclosed bed as in horizontal structural plains. Just as the denuded 

 forms of one cycle of development may be taken as the constructional form 



* High Plateaus of Utah, 1880, p. 19. 



t R. T. Hill has used the term "dip-plains" for the surfaces here called structural plains (this 

 volume, p. 622) ; but as dip is not essential and structure is essential to their production, his term 

 does not seem well chosen. 



