THE WORK OF RUNNING WATER. 



141 



Narrows. — Inequalities in hardness occasion another pecuHarity 

 common to valleys. If a stream crosses vertical or highly' inclined 

 strata of unequal hardness, its valley is usually constricted at the 

 crossing of the harder layers. If such a constriction be notable it is 

 called a narrows, or sometimes Si water- gap (Figs. 121, 159, and Fig. 2, PI. 

 XII). The Appalachian Mountains afford numerous examples. The 

 constriction arises because the processes which widen the valley are less 

 effective on the hard layer than on the less resistant ones on either 



Fig. 121. — Lower narrows of the Baraboo River, Wis. The even-crested ridge is 

 Huronian quartzite. The surroundings are of Cambrian sandstone. (Atwood.) 



Fig. 122.— a hog-back, Jura-Trias. Colorado City, Colo. (Russell, U. S. Geol. Surv.) 



hand. Though most narrows are due to the superior resistance of the 

 rock where they occur, they are sometimes the result of other causes 

 Narrows are much more conspicuous in certain stages of erosion 

 than in others. While a valley is still so young as to be narrow at 



