158 



POPULAR SCIENCE MONTHLY. 



Fia. 1. — Section siiowiNa a Hillside {d c) being 



WASHED DOWN TO THE BaSE PlANE OF ErOSION 



(c 6), WHICH IS itself almost eeduced to Sea 

 Level. 



bility of the formation of any deep valleys. While such finished 

 conditions may often occur over large districts, yet such is not 



the character of very great 

 regions, as Nature seldom 

 allows the completion of 

 these processes, for with 

 the wearing down of one 

 area another quarter is ele- 

 vated by internal forces, 

 until new plateaus rise in 

 bold relief. The rains gather into streams and cut out gorges and 

 valleys with their forms depending upon the character of the 

 rocks and the length of time that the erosion is in progress. The 

 gorges and valleys grow in length, like the Niagara caiion, until 

 the slopes of the streams become so gentle that they can not 

 deepen their channels any more. After that stage, the only work 

 of the river is to carry away the rocks dissolved or washed from 

 the sides of the valleys, which are thus 

 widened into broad flats, and in' their 

 later stages great plains. Such a forma- 

 tion of base planes of erosion is illus- 

 trated in Fig. 2. 



Applying this process of denuda- 

 tion to the lake region, it becomes evi- 

 dent that the land must have stood 

 high enough above the sea for the riv- 

 ers to remove the debris washed into 

 them by the millions of little streams 

 — that is to say, the continent was suffi- 

 ciently high for the excavation of the 

 deepest valleys now beneath the lake 

 waters. As the sea was very distant 

 from the lakes, much farther than now, 

 and the upper lake basins were still 

 farther inland, the altitude of the con- 

 tinent must have been even greater 

 than the depth of the deepest lake basin 

 below the sea level. On the other hand, 

 the slope of the land must have been 



gentle, with the elevation just high enough to allow the drainage 

 of the valleys, without the production of canons through them, 

 and to enable the streams to widen them into broad, rolling hills 

 and plains scores or hundreds of miles wide. (See Figs. 13 and 

 14.) The necessary altitude may have varied from time to time, 

 but the duration of the proper conditions was very long. The 

 elevation was not merely high enough to allow the reduction of 



Fio. 2. — Map of a Plateau beino 



TRANSFORMED INTO A V ALLEY 

 (C C 6'), WHICH IS BROADENING 



OUT INTO A Plain {p p). The 

 sluggish river is only acting as 

 a carrier for the removal of the 

 land washes. 



