354 



GEOLOGY 



were able to develop plane surfaces at some points, surfaces marked 

 by more or less symmetrical drift-hills, which are measurably inde- 

 pendent of rock-topography at others, and short, choppy hills, associated 

 with undrained depressions, in still others. 



The true theory of the drift must explain all these facts and rela- 

 tions. Any hypothesis which fails to explain them all must be incom- 

















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W* 



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*. r..- 



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Fig. 493. — Glaciated dome, Tuolumne Valley, Cal. 



plete at the least, and any hypothesis with which these facts and rela- 

 tions are inconsistent, must be false. 



Geologists are now very generally agreed that glacier ice, sup- 

 plemented by those other agencies which glacier ice calls into being, 

 is the only agent which could have produced the drift. But it is not 

 to be forgotten that this does not preclude the belief that at various 

 times and places, in the course of the ice period, icebergs may have 

 been formed, or that locally and temporarily they played an impor- 

 tant role. It does not preclude the idea that, contemporaneously with 



