12 THE ILLINOIS GLACIAL LOBE. 



From Professor Rolfe's maps estimates have been made of the areas . 

 between each 100-foot contour in each county, and the results combined 

 for the entire State. By these estimates the average altitude of the State 

 is 633.55 feet, or but a few feet different from the estimate made by Mr. 

 Gannett. The average thickness of drift in the State is estimated to be 

 between 100 and 130 feet. (See discussion, Chapter XIII.) If the thick- 

 ness of the drift be deducted, the average altitude of the rock surface in 

 the State will be about 525 feet, or fully 50 feet below the surface of Lake 

 Michigan. The following table gives the estimated areas between 100-foot 

 contours for the entire State: 



Estimated areas in Illinois between 100-foot contours. 



Square miles. 



Above 1,200 feet 1 



Between 1,100 and 1,200 feet 6 



Between 1,000 and 1,100 feet 118 



Between 900 and 1,000 feet 1,009 



Between 800 and 900 feet 3,981 



Between 700 and 800 feet 11,127 



Between 600 and 700 feet 20,058 



Between 500 and 600 feet 9,603 



Between 400 and 500 feet 8,822 



Between 300 and 400 feet 1,925 



Area of Illinois 56,650 



As noted above, the Illinois lobe extended into western Michigan and 

 western Indiana, and also into southeastern . Iowa. Contour maps of these 

 districts have not yet been made, except in portions of Scott and Muscatine 

 counties, Iowa, covered by the United States Geological Survey sheets. 

 The numerous railway surveys have, however, established the altitude of 

 so many points that a fair estimate of the altitude of these districts may be 

 made. It is found that the drift surface has an average altitude slightly 

 higher than that of Illinois, but it will scarcely exceed 650 feet above tide. 

 The rock surface, on the other hand, appears to be a few feet lower than 

 the average for Illinois. The inclusion of these districts, therefore, scarcely 

 makes a measurable change in the average altitude of the region covered 

 by the Illinois lobe. But if the Lake Michigan basin be included, it wil' 

 materially lower the average altitude. 



Profiles across the basin of Lake Michigan have been prepared from 

 Lake Survey charts which bring out certain prominent features of the 

 southern end of the Lake Michigan basin as it now appears with its coating 



