104 



PHYSICAL GEOLOGY 



nental Divide, which separates the basin of the Mississippi River 

 which empties into the Gulf of Mexico and that of the Snake River 

 which finally discharges its waters into the Pacific Ocean, is in the 

 Yellowstone National Park. A divide may be a sharp ridge, as, for 



Fig. 86. — Block diagram illustrating the formation of outliers and the erosion of a 

 plateau. The fronts of the High Plains in Nebraska and elsewhere are being cut back in 

 this way. 



example, in the Kicking Horse River basin of British Columbia, 

 where the divides between the tributaries have been worn down to 

 knifelike ridges which in many places are not a foot in width; or 

 a flat plain, so level that the location of the divide is uncertain. Such 

 a divide is the height of land between the Great Lakes and Hud- 

 son Bay, where the same swamp often drains both north and south. 

 The position of the divide between the Orinoco and Amazon rivers 

 in South America is, perhaps, even more uncertain. Divides are 



seldom stationary, 

 since the streams on 

 the opposite sides do 

 not usually cut head- 

 ward or laterally with 

 equal rapidity. The 

 divide between two 

 tributaries of the 

 same river may also 

 be narrowed by the 

 lateral erosion of the 

 streams until it dis- 

 appears (Fig. 79). 

 By an increase in the number of tributaries, ridges are cut into 

 lulls. In this way the Seven Hills of Rome were sculptured, and 

 many of tin- conspicuous buttes of the western United States 

 (p. 328) were separated from the higher plains (Figs. 86, 87). 



Eagle Rock, Nebraska. (U. S. Geol. Surv.) 



