The Rivers and Valleys of Pennsylvania. 24:5 



northern part of Franklin county, that any water is still drawn 

 from the back of Blue mountain. Again, these small stream gaps 

 do not lie between large river-gaps and wind-gaps, but wind-gaps 

 lie between the gaps of large rivers and those of small streams 

 that are not yet diverted. Excellent illustration of this is found 

 on the " Piedmont sheet " of the contoui-ed maps issued by the 

 United States Geological Survey. The sheet covers part of 

 Maryland and West Virginia, near where the North Branch of 

 the Potomac comes out of the plateau and crosses New Creek 

 mountain. Eleven miles south of the Potomac gap there is a 

 deep wind-gap ; but further on, at twenty, twenty-five and 

 twenty-nine miles from the river-gap are three fine water-gaps 

 occupied by small streams. This example merely shows how 

 many important points in the history of our rivers will be made 

 clear when the country is properly portrayed on contoured maps. 



A few lines may be given to the general absence of gaps in 

 Blue Mountain in Pennsylvania. When the initial consequent 

 drainage was established, many streams must have been located 

 on the northward slope of the great Cumberland highland, C, C, 

 fig. 21 ; they must have gullied the slope to great depths and 

 carried away great volumes of the weak Cambrian beds that lay 

 deep within the hard outer casings of the mass. Minor adjust- 

 ments served to diminish the number of these streams, but the 

 more effective cause of their present rarity lay in the natural selec- 

 tion of certain of them to become large streams ; the smaller ones 

 were generally beheaded by these. The only examples of streams 

 that still cross this ridge with their initial Permian direction of 

 flow to the northwest are found in two southern branches of 

 Tuscarora creek at the southern point of Juniata county ; and 

 these survive because of their obscure location among the many 

 Medina ridges of that district, where they were not easily acces- 

 sible to capture by other streams, 



38. Tertiary adjustment of the Juniata on the Medina anti- 

 clines. — The lower course of the Juniata presents several examples 

 of adjustment referable to the last part of the Jura-Cretaceous 

 cycle and to the Tertiary cycle. The explanation offered for the 

 escape of this river from its initial syncline did not show any 

 reason for its peculiar position with respect to the several Medina 

 anticlines that it now borders, because at the time when it was 

 led across country to the Wiconisco syncline, the hard Medina 

 beds of these anticlines were not discovered. It is therefore 



