J. J. Stevenson—Surface Geology of Pennsylvania. 247 
From the summit-plain of this Brush Ridge, the surface falls off 
in regular steps. 
If, now, the observer turn his attention to the region lying 
directly west from the Monongahela River, he will see that No. 
16 is a broad continuous plain beyond that river, but that still 
farther back toward the west, the fourteenth bench, on an islan 
of which he is standing, forms a similar plain, while stil] farther 
back, No. 18, with an altitude of 1,380 feet above tide, stretches 
_ northward and southward and is broken only by the narrow 
valleys in which the larger streams flow. 
Should the observer's position be changed to Hillsborough, 
fifteen miles west from the Monongahela River, where the ele- 
vation is about 1,500 feet above tide, he will see that No. 13 is 
of great extent north and south, while back of it the country 
rises to a still higher level, again and again, until it reaches No. 
Li at 1,445 feet above tide. 
From the river westward to Hillsborough, or rather to a 
ridge passing nearly north and south at three miles east from 
that village, the surface rises in a succession of steps which are 
beautifully marked. From the hill-top at Hillsborough, the 
descent to the river is very handsomely shown. : 
That these benches are simply the result of remodeling val- 
leys formed long before the agent making the benches began to 
work, is shown by the distribution of the benches themselves ; 
for these benches line the sides of long narrow valleys reaching 
far inland from the rivers, and breaking through ridges bearing 
valleys began, I believe, even before the anticlinal axes had 
been elevated sufficiently to affect the topography. he main 
streams of the present drainage system break through all the 
bol anies of Virginia. 4 
d axes west from the Alleghanies a oy lene faoin their 
condition of preservation. If they had been of ancient origin 
ette county. 
The whole structu re of t 
: : : age iate vicinity. They exten 
ae toeiten died ee 2? ee oot Be the Alleghanies 
