GEOLOGICAL AND NATURAL HISTORY SURVEY. 17 



of their lower courses to beigihts of 75 to 100 feet, or more, above 

 the present streams. But before deposits from the waters, contrib- 

 uted by the succeeding glaciation, were poured into these valleys, 

 they had been cut down about to the present level of the streams 

 and in some cases, perhaps, to an even lower level. 



The cutting down of the valleys indicates a marked change 

 in altitude or at least in attitude of the land surface, and this 

 change cannot have been accompli'sdied in a very brief period. 



EARIiT WISCONSIN DEIFT SHEETS. 



Under this term are included several important morainic sys- 

 tems, which are well displayed in the State of Illinois. The names 

 applied to these morainic systems are in every case taken from 

 Illinois localities. Shelbyville is derived from the county seat of 

 Shelby County, which is situated on the outer belt of moraines. 

 Champaign is from the city of that name in Eastern Illinois, which 

 is situated on one of the principal members of the morainic sys- 

 tem to which the name is applied. Bloomington is situated upon 

 the principal member of a still later system of moraines, well dis- 

 plaved in Central Illinois. Marseilles is situated at a point where 

 tlie moraine to which this name is applied crosses the Illinois 

 River. 



It is thought that the several morainic systems just men- 

 tioned were formed in comparatively rapid succession in the order 

 named. At least no decisive evidence of a prolonged interval 

 separating the formation of successive morainic systems has been 

 discovered. It is probable that some oscillation of the ice front 

 occurred, so that the moraines do not mark simply halts in the 

 recession of the ice. They may mark the limits of successive 

 advances following retreats of minor importance. 



Shelbyville Morainic Svstefti. — The Shelb^wille morainic 

 svstem marks the southern limits of a sheet of thick drift, the 

 thickest, perhaps, formed in Illinois at any stage of the glacial 

 period. The distribution of the moraine may be seen by reference 

 to the accompanying map (Plate 2). Whether the outer ridge 

 throughout the entire extent from the Wabash River, near Terre 

 Haute, Ind., around to the Wisconsin line, near Harvard, 111., was 

 occupied at any one time by the ice sheet, or whether there were 

 oscillations by which some parts were formed earlier than others, 

 is not yet definitely settled. Possibly the outer ridge in the vicinity 

 of the Illinois River should be classed with the lowan rather than 

 the Early Wisconsin drift. This system has not much relief 



