162 



PHYSICAL GEOLOGY 



Narrows. When the rocks in the sides of a valley are 

 of unequal strength, the valley is widened at unequal rates 

 at different points, and if the difference in the character of 



the rocks is great, the valley 

 may become wide where they 

 are weak, while still narrow 

 where they are strong. The 

 places where valleys have 

 much less than their usual 

 width are called narrows or 

 water gaps (Figs. 162 and 163). 

 Delaware Water Gap and 

 Harper's Ferry on the Potomac are among the more famous 

 of many narrows in the Appalachian Mountain region. Nar- 

 rows develop best where there are great differences in the 

 strength of the rocks which form the valley sides, within 

 short distances. They are, therefore, usually associated with 

 highly tilted beds rather than with horizontal ones. They 

 are most conspicuous, also, in connection with mature valleys, 

 for very young valleys are narrow everywhere, and very old 

 valleys have become wide everywhere, regardless of the char- 

 acter of the rocks. 



FIG. 162. Diagram showing water 

 gaps. 



FIG. 163. A typical water gap in the Appalachian Mountains. The 

 Narrows of Wills Mountain at Cumberland, Md. Several roads famous 

 in American history sought the West through this gap. (Md. Geol. Surv.) 



Canons. Valleys that are strikingly deep in relation to 

 their width are called gorges (e.g. Niagara Gorge), dells (e.g. 

 the Dells of the Wisconsin), and especially in the West, 



