440 THE ILLINOIS GLACIAL LOBE. 



responded within a few weeks by sending a shell which proved to be the 

 ordinary oyster. Since the beach marks the border of a lake which stood 

 sufficiently above sea level to maintain a strong- current through its outlet, 

 it seems highly improbable that the lake was occupied by marine life at 

 this time. Upon revisiting the gravel pit and inquiring particularly into 

 the circumstances, it was found that the shell was picked up by some of the 

 workmen near the base of the pit. But it was also found that there are a 

 few Indian graves which extend down nearly to the level of the base of 

 the pit. The shell, therefore, ma}^ have been introduced at the time these 

 graves were made, or may have been of more recent introduction. Remains 

 of terrestrial life have also been found in this gravel pit. Mr. Haas has 

 preserved fragments of the tooth of a mammoth found at the depth of 

 several feet. These fragments are waterworn, and it seems, therefore, quite 

 probable that they were embedded during the formation of the beach 



Another locality in which supposed Unio shells have been reported is 

 found in a marsh on the inner side of the beach north of New Buffalo, 

 Michigan. Mr. Grlavin, formerly county smweyor of Berrien County, 

 rejiorts having observed shells as large as the ordinary clam shell in ditching 

 near the borders of this marsh. He has, however, preserved none of the 

 shells, and possibly may be mistaken in his identification. So far as known 

 to the writer, these are the only places along the entire length of the upper 

 beach where molluscan shells have been reported, and none have been per- 

 sonally found, though search has been made for them in several exposures 

 and excavations. There appears, therefore, to have been a great scarcity of 

 molluscan life in this stage of Lake Chicago. 



INTERVAL OF EMERGENCE. 



After the Grlenwood beach was formed, the lake appears to have with- 

 drawn from the plain in Illinois lying between the beach and the shore of 

 Lake Michigan. To what extent it withdrew within the present limits of 

 the lake is not accurately determined. The evidence for this emergence 

 near Chicago is found in beds of peaty material that occur beneath gravel of 

 the succeeding lake stage, as long since noted by Dr. Andrews and dis- 

 cussed in his paper cited above. Similar peaty material underlies beach 

 gravel near Michigan City, Indiana, In Wisconsin, clay beds which seem 

 to have been left in a retiring water body, and which are covered by beach 



