THE WORK OF RUNNING WATER 



79 



so control lines of travel. The narrows of the Potomac River in 

 Wills Mountain, Maryland, may serve as an example. In the early 

 days of American history, Fort Cumberland was built at the nar- 



Fig. 78. The Lower Narrows of the Baraboo River, Wisconsin. The valley 

 widens out beyond the gap, the same as in the foreground. 



rows to guard the important pass through the mountains, and Wash- 

 ington's and Braddock's Roads ran west through it. At the present 

 time, the Cumberland National Road and an important railway 

 make use of it. 



Rock terraces. Again, if the hard layer through which a stream 

 cuts is horizontal, it weathers away less rapidly than the weaker 



Fig. 79. A butte. A characteristic feature of the arid plateau region of the 

 West. The butte is really a monadnock. (U. S. Geol. Surv.) 



rock above and below, and so gives rise to rock terraces, as shown 

 in Fig. 70. 



Monadnocks, rock ridges, etc. Elsewhere than in valleys, too, 

 hard rock affects the topography, for rain-wash, wind, and most 



