CHAFTEE IX. 



THE PEOEIAN OR POST-LOESSIAL SOIL AND WEATHERED 

 ZONE (TORONTO FORMATION!). 



The soil and weathered zone formed on the lowan till aiid the loess 

 and associated silts before the culmination of the Wisconsin stage of glaci- 

 ation have been called the Peorian/ because of good exposures in the 

 vicinity of Peoria, 111 , beneath the Shelby ville or earliest sheet of the 

 Wisconsin series. The interval between the lowan and Wisconsin stages 

 had pi'eviously been provisionally named Toronto by Chamberlin,^ because 

 of excellent exposures of interglacial fossiliferous beds along the Don Valley 

 in Toronto, Ontario, which were at first thought to be of this age. 

 Chamberlin remarks, in connection with the introduction of this name, that 

 the grounds for the correlation are not very strong, and that further investi- 

 gation may show them to be erroneous. In view of the uncertainty attached 

 to this correlation it has seemed advisable to employ for the present a sub- 

 stitutional name which is known to be applicable to the interval between 

 the lowan and the early Wisconsin. In case the correlation suggested by 

 Chamberlin is demonstrated to be correct the name Toronto has precedence. 



The evidence of this interglacial interval is found not only in the 

 formation of a soil and leached horizon at the top of the loess, but also 

 in a great change in the outline of the ice sheet in the succeeding or 

 Wisconsin glaciation from that displayed in the preceding or lowan 

 glaciation. There was also a marked change in the attitude of the land, 

 the conditions for drainage being decidedly better in the Wisconsin than 

 in the lowan stage of glaciation. 



These several lines of evidence are well shown in the region covered 

 by the Illinois glacial lobe, and are discussed in Monograph KXXVIII. 



^The Peorian soil and weathered zone (Toronto formation?), by Frank Leverett: Jour. Geol., 

 Vol. VI, 1898, pp. 244-249; see also Mon. U. S. Geol. Survey, Vol. XXXVIII, 1899, pp. 185-190. 



2 Classification of American glacial deposits, by T. C. Chamberlin; Jour. Geol., Vol. Ill, 1896, 

 pp. 270-277. 



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