GEOLOGICAL AND NATURAL HISTORY SURVEY. 71 



found, and this negative evidence strengthens the view that the 

 shells at this pit may be intrusions. Remains of terrestrial life have 

 also been found here. Mr. Haas preserved fragments of the tooth 

 of a mammoth found in the gravel pit at a depth of several feet. 

 These fragments are waterworn, and it seems, therefore, quite 

 probable that they were imbedded during the formation of the 

 beach. 



Another gravel pit has been opened in the beach between 

 Salt Creek and LaGrange, in which the excavation extends from 

 the east side of the beach westward nearly to the outer slope. It 

 has a depth of 12 to 14 feet in the deepest part, and exposes a series 

 of beds dipping slightly toward the east. The upper five feet is of 

 brown stained gravel. The lower portion is a fine gravel, with 

 very little stain. In the gravel there are sandy pockets and also 

 thin beds of sand. These sandy portions, in some cases, show a 

 slight effervescence with acid, but are not nearly so calcareous as 

 the sand found at similar depths in glacial deposits. 



Interval of Emergence. — After the Glenwood beach was 

 formed the lake appears to have withdrawn from the plain between 

 the beach and the shore of Lake Michigan, in Illinois. How much 

 farther north it withdrew is not accurately determined. Professor 

 Chamberlin recognized evidence of emergence between the forma- 

 tion of this beach and the second beach, in Southeastern Wisconsin. 

 That it withdrew so much in the northern end of the Lake Michi- 

 gan basin as in the southern seems improbable from the evidence 

 drawn from tilting, it being found by Mr. F. B. Taylor that that 

 portion of the basin has been uplifted rnore than the southern. 

 Whether this emergence is to be connected with the lake stage, 

 known as the Algonquin, is not yet ascertained, though that seems 

 a probable correlation. 



The evidence for this emergence within the Chicago area is 

 found in beds of peaty material that occur beneath gravel of the 

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

 discussed in his paper cited above. In Wisconsin the evidence is 

 in clay beds, which seem to have been left in a retiring water 

 body, and which are covered by beach deposits of the succeeding 

 lake stage, as pointed out by Prof. Chamberlin in his official report 

 of the Wisconsin Geological Survey. There is need for further 

 study of this interval in the lake history before conclusions of 

 much consequence can be drawn. 



