HISTORICAL REVIEW 21 



been carried on especially relating to this subject. The lack of our knowledge 

 of this subject is probably due to the fact that the majority of the workers and 

 students n this field have been geologists interested mainly in the stratigraphy 

 and physiography of the area. That a systematic study of other parts of this 

 and other regions would produce notable additions to our knowledge is evi- 

 denced by the results obtained by the writer from a systematic examination 

 and study of the deposits exposed in the drainage ditches and other excavations 

 made in t he Chicago Area, which are described in the following chapter. 



