GEOLOGICAL AND NATURAL HISTORY SURVEY. 51 



the line of the old outlet form a large part of the section. (See 

 Figs. I and 2.) 



In the Fullerton avenue conduit which leads eastward into the 

 lake through the north part of Chicago the drift is mainly till, but 

 the surface sand is a conspicuous deposit. From its western end 

 to within 2,000 feet of the lake the rock surface is found at a depth 

 of 43 to 54 feet. Within 100 feet east from this point it drops down 

 to 80 feet, passes below the conduit, and does not appear further 

 east. The sand has its greatest thickness at about 1,700 feet from 

 the lake, where it reaches 25 feet. It decreases westward to 17 feet 

 at 2,100 feet from the lake, and to 12 feet at 6,000 feet, and entirely 

 disappears before reaching the Chicago River valley. Toward the 

 lake shore also it decreases, being about 18 feet in depth for 1,400 

 feet west from the shore. At the water's edge the depth is but 10 

 feet. The profile continues out 1,100 feet below the lake, and there 

 is but three feet of sand at its terminus. 



The profile of the lake tunnel shows the rock to be at a depth 

 of 2)7 feet at the working shaft four miles inland. Eastward from 

 here the distance to rock gradually increases to a point about two 

 and four-tenths miles from the lake shore, being between 50 and 

 60 feet at that point. It there drops down rapidly to a level nearly 

 90 feet below the surface, and is not encountered farther east. For 

 two miles out beneath the lake the excavation reaches 60 feet or 

 more below the lake bottom. The tunnel was through till its 

 entire length. It shows, as noted above, pockets of sand inclosed 

 in the till. The surface sand, as in the Fullerton avenue conduit, is 

 of considerable depth. 



In Hyde Park Township in the south part of Chicago numer- 

 ous borings and excavations show the sand deposits to be from 

 five feet up to about 20 feet in depth. Except where the rock 

 comes near the surface till is found to underly the sand. 



