410 THE ILLINOIS GLACIAL LOBE. 



heir whose width varies from scarcely 2 miles up to fully 5 miles. Where 

 the dimes are highest the sand probably has a thickness of about 200 feet, 

 for the dunes attain a height of 150 to 175 feet, and the sand, as shown by 

 wells, extends a few feet below the lake level. Throughout much of the 

 belt the sand probably exceeds 25 feet in depth and may average twice that 

 amount. South of Calumet River, from the mouth of Salt Creek, near 

 Chrisman westward past Lake and Liverpool to Griffith, there is a belt of 

 sand a mile or more in width which has generally a depth of about 20 feet. 

 East from Salt Creek the depth is much less. The sand is also of slight 

 depth west from Griffith except along the line of the beaches. 



Beneath this heavy deposit of sand there appears to be but little oxi- 

 dized clay, a feature which favors the interpretation that the clay was not 

 long exposed to atmospheric action before the sand deposition occurred. 

 The wells usually pass immediately into a blue clay. This clay, so far 

 as can be learned from well drillers, is but slightly pebbly and appar- 

 ently is in places free from pebbles. It seems to maintain this character 

 to great depth, as is indicated by sections of wells already given. It differs 

 markedly from the blue till of the neighboring portion of the plain ii: 

 Cook County, Illinois, and appears also to be somewhat less pebbly than 

 the blue clay of the neighboring district in southwestern Michigan. There 

 are few exposures afforded by the streams in the district where sand is 

 heavy, but exposures of slight depth are numerous outside the limits of 

 the heavy sand. From these exposures it appears that the clay has gener- 

 ally but few pebbles, and several exposures have been found in which it is 

 pebbleless. 



The most extensive exposures of pebbleless clay noted are along - Deep 

 River, in the vicinity of Hobart, and it appears to be present over an area 

 of several square miles between Deep River and Salt Creek. Prof. W. S. 

 Blatchley, State geologist, reports a similar clay at Chesterton and Michi- 

 gan City. 1 This pebbleless clay is oxidized to a depth of a few feet, 

 beneath which it presents a blue color similar to that of the pebbly clay of 

 neighboring districts. It is highly calcareous and carries numerous lime- 

 stone nodules near the bottom of the oxidized portion. It seems even more 

 calcareous than the pebbly blue clay. Professor Blatchley has published 



1 Communicated to the writer. 



