130 THE ILLINOIS GLACIAL LOBE. 



Carroll, Illinois. At this place the loess has a thickness of nearly 20 feet. 

 The till beneath this buried soil has been leached as far down as exposures 

 extend, 3 to 4 feet. Still another extensive exposure of the soil is found in 

 the pit of the Brick and Tile Works at Galva, Illinois. The loess, to a 

 depth of 15 feet, is used in the manufacture of the brick and tile, beneath 

 which is a black mucky soil about 1 foot in depth, which caps the Illinoian 

 till sheet. In this soil a log about 1 foot in diameter and several feet in 

 length was found embedded. 



The conditions for erosion during the Sangamon interglacial stage seem 

 to have been decidedly less favorable than in the Yarmouth stage. The 

 streams apparently were so broad and sluggish as to cut only shallow val- 

 leys, and these are seldom sharply outlined. The general absence of 

 well-defined valleys beneath the Iowan loess on the area occupied by the 

 Illinoian drift sheet, when taken in connection with their conspicuousness 

 in the Kansan drift, is liable to give the impression that only a brief inter- 

 val separates the Illinoian from the Iowan glacial stage. But if interpreta- 

 tions are made from the leaching, and depth of peat and muck accumulation 

 during the Sangamon interglacial stage, there are found indications of a 

 period which compares favorably in length with the Yarmouth interglacial 

 stage. The depth and degree of oxidation of the Illinoian, when compared 

 with the Iowan, are also impressive evidence in favor of a wide separation 

 in the dates of deposition of the two deposits. The amount of erosion, 

 therefore, appears to be a poor index of the length of the interglacial period, 

 though if low altitude and slack drainage be assumed it easily harmonizes 

 with the evidence of a long interval denoted by the other features of the 

 drift. In discussing this matter with the several glacialists who are familiar 

 with the deposits representative of each glacial stage the writer finds them 

 unanimous in considering the Illinoian drift a much older deposit than the 

 Iowan. There is some difference of opinion as to whether the Sangamon 

 or the Yarmouth is the longer interglacial interval. The writer inclines to 

 the opinion that the Yarmouth is the longer interval. 



