356 THE ILLINOIS GLACIAL LOBE. 



STRUCTURE OF THE DRIFT. 



The drift of the Valparaiso sheet is found to show interesting changes 

 in structure when followed around the head of Lake Michigan. In the 

 Illinois portion it consists mainly of a soft blue till, similar to that forming 

 the bulk of the moraines in the early Wisconsin series in that State. Upon 

 passing eastward into Indiana till predominates over sand and gravel as far 

 east as the meridian of Valparaiso. From this line northeastward there is 

 a decided change in constitution, and sand and gravel predominate over till. 

 The prominent portions of the morainic system appear to contain a larger 

 proportion of sand and gravel than the lower portions. This is true not 

 only of Porter and Laporte counties, Indiana, but of Berrien, Van Buren 

 and Allegan counties, Michigan. It is not rare to find well sections in these 

 prominent portions of the morainic system which penetrate nothing- but 

 sand and gravel in a depth of 100 feet or more. Till, however, is usually 

 present in small amount on these elevated tracts. In Lake and Porter 

 counties, Indiana, the till has considerable depth on the north or inner 

 border of the morainic system, but is underlain at slight depth by sand and 

 gravel on the southern border of this system, thus forming a phase inter- 

 mediate between the gravelly, sandy portion to the east and the till portion 

 to the west. 



The cause for the marked difference in constitution displayed by this 

 morainic system can as yet only be conjectured. The writer's attention was 

 called by Mr. F. B. Taylor to the coincidence of the gravelly and sandy 

 portion of the morainic system with the extent of prominent dunes along 

 the shore of the lake, and the suggestion was made that the g'reat abun- 

 dance of sand in this portion of the moraine may be due to the incorpora- 

 tion of dune material which had been blown into that region during an 

 interglacial interval. Some doubt concerning - this view is raised by the 

 absence of sandy material in the other moraines formed on the southeast 

 border of Lake Michigan. This, however, may not be a fatal objection, 

 since the invasion which formed the Valparaiso moraine may have gathered 

 up the entire deposit of dune material and may have been followed so closely 

 by the deposition of the later moraines that no opportunity for the forma- 

 tion of dunes was afforded. In the writer's opinion a cause for the sandy 

 and gravelly constitution may be found in the relation of the Lake Miclii- 



