THE TEMPORARY MISSISSIPPI RIVER 429 



Although known to Calvin'^ as early as 1874, partially mapped 

 and described by Udden^ in 1899, and noticed by Leverett^ in the 

 same year, conclusive evidence of the existence of Lake Calvin 

 had not been presented until the writer^ completed his studies on 

 the origin and history of extinct Lake Calvin in 19 19. 



ILLINOIAN GUMBOTIL 



So long as Lake Calvin existed, the Mississippi must have 

 followed the temporary course outlined on the previous pages. 

 Hence any evidence favoring a long life for the lake must favor at 

 the same time a long duration for the temporary Mississippi River. 



The duration of Lake Calvin is intimately related with the forma- 

 tion of the lUinoian gumbotil. For, as it will be shown, the lake 

 could not have been drained until after the gumbotil was formed 

 and the valleys of Iowa-Cedar and Mississippi rivers were developed. 

 It has been shown by Kay^ that in the formation of any gumbotil 

 a very long period of weathering of the till is required and that 

 during this period of weathering erosion is very slight. That the 

 formation of the gumbotil is an exceedingly slow process is indicated 

 by the fact that no gumbotil is found on the lowan and Wisconsin 

 drift sheets which are believed to be too young to have had a gum- 

 botil developed on them.^ The Illinoian gumbotil is on the average 

 5 feet thick. 



During the very slow process of weathering of the till to form 

 the Illinoian gumbotil, erosion was very slight. It is inconsistent 

 with the theory of the formation and origin of the gumbotil that 

 vaUeys sufficiently deep to drain the lake would have permitted 

 the formation of the Illinoian gumbotil to the very edges of the 

 valley walls. Especially is this true as the gumbotil lies in a hori- 

 zontal plane and does not conform to the surface slopes produced 



^ J. A. Udden, op. cit., pp. 352-53- 



2 Op. cit., pp. 246-388. 



3 F. Leverett, "The Illinois Glacial Lobe," U.S. Geol. Survey Monograph XXXVIII 

 (1899), pp. 89-97. 



'' W. H. Schoewe, "The Origin and History of Extinct Lake Calvin, Iowa," Iowa 

 Geol. Survey, Vol. XXIX, 1919. (In press.) 



5 G. F. Kay, and J. N. Pearce, "The Origin of Gumbotil," Jour, of Geol., Vol. 

 XXVIII (1920), pp. 89-125. 



^Op. cit., p. 124. 



