228 THE ICE AGE IS SOUTH AMERICA. 



leys of erosion. Among the best-known examples in the 

 country are those of the Mohawk and Hudson in Xew York : 

 of the Delaware and Susquehanna in Pennsylvania ; the 

 Ohio and its tributaries on the western flanks of the Alle- 

 ghanies ; the Mississippi and all its western branches, together 

 with the Colorado upon the eastern flanks of the Rocky 

 Mountains ; and the Columbia, the Fraser, and the Stickeen 

 Rivers, which penetrate in chasms of great depth the rock- 

 bouud shore of the Pacific. 



At the Delaware Water-Gap the river has sawn a vertical 

 chasm more than a thousand feet deep directly across the 

 hard strata of the Kittatinny Mountain. For fifty miles 

 above Lock Haven, on the West Branch of the Susquehanna, 

 the river occupies a narrow valley of erosion more than a 

 thousand feet in depth. For nearly twelve hundred miles, 

 as the water runs, the Ohio River, with its extension up the 

 Alleghany, occupies a narrow, crooked valley, one mile or 

 more in width and several hundred feet in depth, which it 

 has worn through nearly parallel strata of lime and sandstone 

 rock. The trough of the Mississippi from Cairo upward is 

 similar- to that of the Ohio, except that it is two or three 

 times as broad. The canons of the Colorado, of the Yellow- 

 stone, and of the Columbia, are of world wide renown. The 

 Colorado has worn a channel with nearly perpendicular sides 

 three hundred miles long and from three to six thousand 

 feet deep.* Such are some of the well-recognized results 

 produced by the long-continued mechanical action of running 

 water. 



But, aside from its mechanical action, water with the 

 acids it contains is a most efficient chemical force, acting as a 

 solvent upon various rocks. Every salt-spring and every 

 spring of hard water in a limestone region is undermining 

 the country from which it issues, and is engaged in dissolving 

 the solid material and in transporting it in solution to lower 

 levels. It is only a question of time when the chalk cliffs of 



* "Elements of Geology," by Joseph Le Conte, pp. 15-17. 



