76 V. p. GULLIVER PLANATION AND DLSSECTION OF URAL MOUNTAINS 



off makes its cliff origin seem doubtful, and the old forui of the valleys 

 and the decomposed rock in ])lace point toward an old land surface. 



If this low crystalline plain to the east of Ilmen mountain is an old 

 land surface worn down, its relation to the summit-level ])lane of the 

 Urals must be explained. It surely could not have been formed as the 

 two plains stand today, and if the two were once continuous there must 

 have been a fault along the eastern base of Ilmen mountain after the 

 completion of the planation. If this hypothesis prove to be the correct 

 one, then this Ilmen fall-off will be a very fine example of a fault scarp. 



As bearing on the hypothesis of faulting after planation, it must be 

 mentioned at this point that there were found by the present writer at 

 several places on the slopes of Ilmen mountain benches which accord in 

 elevation, as nearly as could be made out, with the grade-plains found in 

 the central portions of the Urals, which were no doubt formed by streams 

 after the uplifting of the nearly completely planed mountains. 



If in the faulting there had been a slight tilt of the crystalline area to- 

 ward the west, then the streams in this area would be hindered and in 

 manj^ places ponded. It should then follow that the lakes would be 

 more abundant where the summit-level plane falls off more steepl3\ 

 This is a question for further field study. 



VALLEYS IN THE URALS 



Beneath the summit-level plane the streams have carved their valleys, 

 deeper in the central portion of the mountains where the planed land 

 surface had been uplifted to the greater amount, and shallower where 

 there had been but little up-arching, slower in more resistant strata, and 

 faster in the weaker ones ; but the striking feature of all the vallej's is 

 that they are not due to one simple uplift, but that they have been de- 

 veloped during a series of successive up-archings. 



Planation 



work of rivers 



The logical conclusion from a study of the weathering of rocks and 

 the transportation of rock waste is that in any place the ultimate and 

 necessary result of such degradation of its surface must be the reduction 

 of the land to a nearly uniformly level plain close to sealevel. Powell, 

 Button, and Gilbert were the first to clearly state this law of land base- 

 leveling, after their studv of the widening of the deeply cut can^^ons of 

 the Colorado plateau. At Harvard Prt>fes.sor Davis has worked out with 

 his students man}^ of the characteristic features of land form connected 

 with this i)rocess. The controlling plane beneath which tlie river can 

 not cut is the level of the sea at the mouth of the river; the great plane 



