18 PHYSICAL GEOGRAPHY OF 



The same considerations apply also to Tygart's Valley, of which the general 

 direction coincides very nearly with that of the Monongahela — both running 

 parallel with the Ohio, but in the opposite direction. But Tygart's valley lies 

 east of the Monongahela, and higher up on the great slope, while it breaks 

 through the ridge of Laurel Hill, and follows the valley lying between that 

 ridge and Cheat mountain. Its fall is therefore greatly increased, and amounts 

 to 150 feet in the first 20 miles above its junction with the Monongahela, or 7^ 

 feet per mile.* 



Cheat river, a more important stream than Tygart's Valley, lies still further 

 east, and runs nearly parallel with that stream as well as with the 3Iononga- 

 hela, in a valley enclosed between the Cheat and Greenbriar mountains. The 

 fall of this stream is 600 feet in the first 47 miles above its confluence with the 

 Monongahela.* 



These mountain ridges and intermediate valleys, and their streams, all run 

 parallel with the crest of the Alleghany, and all rest on the great slope which 

 descends from the summit of that mountain ridge towards the Ohio. 



The summit of the Alleghany, at the lowest passes near the sources of the 

 Greenbriar and Cheat rivers, may be stated at - '2,400 ft. above tide. 



The surface of Cheat river, near the northwestern 



turnpike, at ------- - 1,375 " " " 



The surface of Tygart's Valley, in the same latitude, at 1,000 " " " 



ThesurfaceoftheMonongahela, in the same parallel, at 910 " " " 



The surface of the Ohio, above Parkersburg, at - - 570 " " " 



Descending from the Alleghany to the west, we thus find each stream we 

 cross, in the same parallel, occupying a lower level, until we reach the Ohio, at 

 the foot of the slope. The breadth of this great plane, from east to west, is very 

 uniformly 125 miles, and the total descent in that distance about 1,800 feet, or 

 at the average rate of 14^ feet per mile. 



If we now turn to the opposite plane in Ohio, and assume for the level of its 

 origin the summit which separates the waters which flow into Lake Erie by the 

 Sandusky, from those of the Scioto, or 923 feet above tide, and for that of the 

 foot of the slope the level of the Ohio at Portsmouth, we shall again find the 

 distance about 125 miles, but the descent only 450 feet, or S/t feet per mile. 



The average inclination of the great plane, from the east and south, is, there- 

 fore, four times as great as that of the great plane from the north. 



The Ohio river occupies middle ground in the great valley which it drains, 

 and the waters which it serves to shed are supplied in very nearly equal quanti- 

 ties by its northern and southern slopes. 



* Surveys of Benjamin H. Latrobe, civil engineer. 



