

PHYSICAL GEOGRAPHY OF THE COUNTY. F 2 . 21 



rapidly where it crosses the intermediate valleys. Behind 

 or north of each mountain it is dammed back upon the softer 

 rocks, and descends over the hard rock-ledges in the gap as 

 over a natural wier. 



For example, its rate behind the Buffalo mountain is 1.80', 

 but in the gap itself 3. 57'. 



Belt hid Berry's mountain its rate is 2.66', bat in the gap 

 itself 4.01'. 



Behind Peters' mountain its bed is almost a dead level 

 for four miles, and then falls in the gap at the rate of 7.13'. 



Across the Cove its rate is only 1.00' and 1.58', but through 

 the Second mountain and Blue mountain gaps it averages 

 2.69'. 



Hence to Harrisburg the rate is 1.35'. 



It is plain to see that the ribs of sandstone rock which 

 form the several mountain crests and descend steeply (some- 

 times vertically) to and across the bed of the river in the 

 several gaps, form natural dams (and must have always 

 formed natural dams) over which the river has been tumb- 

 ling for ages. These natural dams, being still in process of 

 destruction, must have been higher above ocean level in each 

 preceding age as w T e follow the process of wear and tear 

 backwards in the order of time. 



It follows that the river, as a whole, must have once flowed 

 on a level with what is now the tops of the mountains. 



But it does not follow that the relative heights of the 

 mountains and valleys were the same then which they are 

 now, and that the whole country behind the Blue mountain 

 was then flooded by the river ; in other words, that all cen- 

 tral Pennsylvania was then a great lake. On the contrary, 

 what is true of the Susquehanna is true as we shall see of 

 the Juniata and all its other main branches, and of all their 

 secondary branches, and of all the creeks and runs and rills 

 which feed these secondary branches with water; for the 

 whole surface of the country must have shared in the 

 gradual lowering of the bed of the Susquehanna. 



But if this be true, then the entire surface of the country 

 must have stood higher and higher above sea level as we go 

 back in time : not the valleys only, but the mountains also ; 



