564 



THE ICE AGE IN NORTH AMERICA. 



Another most instructive illustration of the extent of 

 preglacial erosion is found west of Keokuk, Iowa,* where there 

 is a buried channel of great width now filled with glacial debris 

 while the river at Keokuk flows over a rock bottom and 

 through a comparatively narrow channel. 



Fig. 145 — Meanderings of Plum Creek through 5000 feet of its trough. 



51 Ft. 



- - 2 m." 



» 10 Ft. 



17 Ft-- - 



Fig. 146— Cross section of the new course of Plum Creek, showing its original width and 

 its enlargement in twelve years. 



Another means of measuring thhe amount of erosion 

 since the Glacial period is found in post-glacial river-valleys 

 by estimating the amount of material which has been car- 

 ried out by the present streams from the glacial deposit 

 itself. 



Professor Hicks, of Granville, Ohio, reported in 1884, t 

 some important results of such an investigation in the valley 

 of Raccoon Creek, Licking county, near the glacial border. 

 Thi present flood plain of this creek is now bordered on 

 either side by gravel terraces about fifty feet high, which are 



* See cut on page 310. 



t "Baptist Quarterly" for July, 1884. 



See also Fig. 99, p. 324. 



