20 THE ILLINOIS GLACIAL LOBE. 



divisions to which Chamberlin has applied names appear to have the rank 

 of the main divisions of the Glacial period, whatever that rank may prove 

 to 1 >c Other names are necessary to denote the subdivisions. In the present 

 report several names are thus introduced to designate the moraines associ- 

 ated with the older as well as the newer sheets of drift. The name selected 

 is usually that of a town located on the moraine. In most cases the names 

 have come into use in the office and in correspondence with other glaeialists, 

 as ;i convenient form of reference. The selection thus made seems suitable 

 for general use. 



In the outline given below it is aimed to cover the events between the 

 deposition of the oldest-recognized drift sheet in North America and the 

 final recession of the ice sheet or sheets into the region north of the Great 

 Lakes. The main divisions appear to be much longer than the secondary 

 ones. It is concerning the former that the value of time intervals is a mat- 

 ter of dispute. The secondary divisions of the drift deposit are not 

 thought by any students to be marked by intervals sufficiently prolonged 

 to merit the application of the term epoch. It is probable, however, as 

 shown farther on, that some oscillation of the ice front occurred, so that the 

 moraines on which these subdivisions are based do not mark simply halts 

 in the recession of the ice, but rather readvances after recessions of minor 

 consequence. To. avoid the use of the controverted term epoch, it is thought 

 best to employ the term stage, which gives a less definite time value. 



Outline of the drift sheets and intervals. 



Stage 1. Oldest recognised drift sheet— the Albertan of Dawson, including, also, the sub-Aftoniau 



of Chamberlin. 

 Stage 2. First interval of deglaciatiou— Aftonian of Chamberlin. 

 Stage 3. Kaiisau drift sheet of the Iowa geologists. 



Stage 1. Second interval of recession or deglaciatiou — Yarmouth of Leverett. 

 Stage 5. Illinoian drift sheet. 



Stago 6. Third interval of recession or deglaciatiou — Sangamon of Leverett. 

 Stage 7. Iowan drift sheet and main loess deposit. 

 Stage S. Fourth interval of recession or deglaciatiou — Peorian of Leverett, possibly equivalent to the 



Toronto Formation of Chamberlin. 

 Stage !). Early Wisconsin drift sheets. 



Substage 1. Shelbyville morainic system | S^t =£. 



( Outer ridge. 



Substage 2. Champaign morainic system < Middle ridge 



( Inner ridge. 



( Bloomington or outer main ridge. 

 Substage 3. Bloomington morainic system •' Normal or iuner main ridge. 



( Subordinate ridges. 



Substage 4. Marseilles morainic system ) ^^Sattll ridge. 



