﻿172 J. LeConte — Elevation of the Sierra Nevada. 



widened out. Then during the later Quaternary (Champlain 

 epoch of Dana) the land subsided and the rivers seeking their 

 base-level by deposit rilled up the old channels to their brims. 

 Then lastly during the transition to the present (Terrace epoch) 

 the land rose again and the rivers began again to cut ; but the 

 late Tertiary elevation has not yet been reached by the land 

 nor the old beds by the rivers. 



In the case of the Mississippi river we find as I think the 

 record of still another movement in the opposite direction, i. e. 

 subsidence, in still more recent times. This is shown by the 

 upbuilding again of the river-bed by deposit of recent alluvium 

 to an average depth, according to Hilgard, of 50 feet. Fig. 4 

 is an ideal section across the Mississippi showing these facts. 

 After what we have said the record of this river is easily read. 



1—7^ 



a 



h 





4. 



T 





tM 



It 







IB5 



a 











-.--> 











:/ 



Ideal section across the Mississippi river and bluffs, r r, Present river bed ; 

 r' r' r', old river bed ; a, country rock ; b b, old river sediment. 



1. The old bed r' ' r' V ', 100 miles across, was formed during a 

 more elevated condition of the land in late Tertiary and early 

 Quaternary (Glacial) times. 2. In later Quaternary (Champlain) 

 this was filled to the brim with deposit 400 or more feet thick, 

 because the land was then subsided and probably also because 

 of more abundant sediment. 3. During the Terrace epoch the 

 land rose and the river cut until the present wide channel r r, 

 10 miles wide and bounded by bluffs 200 feet high, was 

 formed. This wide channel is not the result, however, of ordi- 

 nary cliff recession as in the case of the Grand Canon, for there 

 has been neither time enough to produce such width nor suita- 

 ble material to develop by recession such perpendicular forms ; 

 but it is due to the shifting of the great river from side to side 

 and undercutting the bounding bluffs. 4. Lastly, during the 

 present epoch there has been some subsidence again and a re- 

 filling to the extent of 50 feet by the alluvium. Other evidences 

 of this last subsidence are found in old buried forest grounds 

 in the delta, far below the present sea-level. 



The phenomena of European river-beds are substantially 

 similar to those of eastern North America in the drift-region. 



River-beds of California and the evidences of Sierra- Elevation. 



All that I have thus far said is introductory to this the main 

 subject. It is well known since the publication of Whitney's 



