932 GEOLOGY 



in central Kansas and Nebraska, hear an aspect of pronounced topo- 

 graphic youth, suggesting that they have been lying, until recently, 

 near the neutral horizon between erosion and deposition, and have 

 lately been raised on the western side. In the Dakotas, there an- 

 broad gradation plains of abandoned river-courses which cross the 

 present valley of the Missouri River. Their present gradients, and 

 their elevation above the present river-bottoms of the region imply 

 a westward elevation. These and collateral phenomena, taken 

 with the remarkable movement of the Keewatin ice-sheet from 

 what is now the lower to what is now the higher side of the plains, 

 seem best satisfied by the view that until about the close of the 

 Glacial Period the western side of the Great Plains was lower than 

 now, relatively, or the eastern side higher. On the western side 

 of the continent there is much evidence of recent movement, some 

 of which appears to have taken place since the close of the Glacial 

 Period, as usually defined. Similar phenomena are found in other 

 continents. 



It is not wholly clear, therefore, whether the present is to be 

 regarded as a part of that period of deformation which had its 

 climax in the Pliocene, of whether it is rather the initial stage of a 

 period of quiescence now being entered upon. 



The suggestions of existing physiography. The view that the 

 earth is now passing into a period of quiescence is strengthened by 

 the present physiographic features of the earth's surface. These 

 are an inheritance from the Tertiary deformations superposed upon 

 pre-existing configurations, though they have been modified by 

 gradational agencies since. They should tell us whether the face 

 of the earth is that of a planet in the midst of deformation, or that 

 of one recently deformed, and now in a more quiescent stale. 

 Every stream should show whether it has just been rejuvenated. 

 or has done some notable work since it was rejuvenated. Kvery 

 coast should show whether the continental border stands forth in 

 the manner characteristic of an earth-segment just crowded up by 

 a deformative thrust, or whether it has made some not able pro- 

 in settling back, or in being cut back, to an inter-deformative state. 



.Most of the streams of the continents show that they have had 

 time to do some appreciable work since they wore rejuvenated. 



