SAGINAW LOBE. 127 



STRUCTURE OP THE DRIFT. 



The depth to rock is only 40 to 50 feet in the northeastern part of the moraine from near 

 San Pierre to Kankakee River, but increases to nearly 100 feet within a few miles south of San 

 Pierre and on the high part of the moraine north of Rensselaer has not been reached by some 

 wells nearby 200 feet deep. The general depth' from Rensselaer westward is about 125 feet. 



The basal portion of the drift is an indurated till, 35 to 40 feet thick, probabby of pre-Wis- 

 consin age. Above it is a very clayey, easily penetrated till, which is thought to be Wisconsin drift, 

 and which forms the body of the ridge and gives it relief above the bordering plains. The surface 

 portion to a depth of 5 to 20 feet differs markedly from the underlying clayey till, showing many 

 abrupt changes from sand to clay. The sand has a somewhat singular distribution, capping 

 many of the knolls to a depth of a few feet and being absent from many of the contiguous lower 

 tracts. Northward from the ridged portion of the moraine across the bowlder-strewn plain sand 

 patches become more and more noticeable until they coalesce in a continuous sheet. Bowlders 

 abound in portions of the ridged part and are common over the entire ridge; they occur, how- 

 ever, in greatest number on the plain north of the ridge. Most of them are of granite and other 

 crystalline rocks of Canadian derivation, but many are limestone fragments of local derivation. 

 In an excavation at Brook, just south of the moraine, a collection of pebbles 1 J inches or less 

 in diameter, taken in order as they appeared on the slope, was made up of the following rocks : 



Pebbles in till near Brook, hid. 



Limestone, chiefly gray 56 



Shale, sandy 4 



Chert, probably from the limestone 3 



Quartz 2 



Granite 6 



Greenstone 8 



79 



Several wells along the ridge and in the border plain pass from soft, easily excavated till 

 of Wisconsin age to hard, partly cemented till, probably of Illinoian age. North of Rensselaer, 

 in sec. 30, T. 30 N., R. 6 W, two wells reach the bottom of the soft drift at 90 feet, and a third 

 at 97 feet. The first two were extended through a hard till to water-bearing sand at 130 and 

 147 feet, but the third found water between the two sheets of drift. South of the moraine, in 

 the vicinity of Brook, a soft till extends to a depth of 50 or 60 feet, below which a harder till 

 extends either to the rock or to water-bearing beds. About 5 miles northwest of Kentland, in 

 sec. 13, T. 27 N., R. 10 W., four wells on the farm of J. V. Speck are reported to have passed, 

 at about 80 feet, through a brown swamp muck representing an old land surface at the bottom 

 of the till, beneath which they entered water-bearing sand and gravel. In the vicinity of 

 Donovan, 111., several wells pass, at about 100 feet, from soft into hard till, below which, at 

 about 140 feet, lies black muck, underlain by another till sheet. Records of several wells 

 along the line of the ridge make no mention of a change from soft to hard till at depths corre- 

 sponding to those just reported, but this lack may be due to less close observation. The thick- 

 ness of the drift approaches 200 feet in some of the wells. 



A well on Mr. Peterson's farm near the State line, in sec. 11, T. 28 N., R. 10 W., on a high 

 part of the moraine, penetrated 168 feet of drift, of which the lower 8 feet was water-bearing 

 sand and the remainder till. A neighbor's well 171 feet deep passed through 165 feet of till 

 before striking water-bearing sand. 



At Morocco, on the north slope of the moraine, a boring at the creamery penetrated 127 

 feet of till to limestone. Several wells near the range line east of Morocco enter rock at 110 to 

 120 feet after penetrating a solid bed of till. 



At Mount Ayer, also on the moraine, a few wells put down to 140 to 160 feet extend but 

 little into rock. The drift in Mr. Ashby's well has the section given below, Lewis Marion's 

 strikes shale rock at 139 feet, and W. J. Young's at 150 feet, the wells being mainly through 

 till to these depths. 



