CHAPTER III 



THE STORY OF OUR ROCK FOUNDATION 



$i. 



HE rock immediately underlying the soil 

 in the Chicago area is the Niagara lime- 

 stone. It has a thickness of 250-400 

 feet. It is exposed in many places about 

 the city as in the quarries at Stony Island, 

 Thornton, Hawthorne, Elmhurst, on the 

 Chicago & North Western Railway; 

 AlcCook, on the Atchison, Topeka & 

 Santa Fe Railway; and Lockport, on the Chicago & Alton 

 Railroad. There are mile-long heaps of it beside the drainage 

 canal from which it was excavated from Willow Springs to Lock- 

 port, and also beside the branch ^anal from Blue Island to the 

 Sag. At Lockport the rock comes to the surface and forms the 

 bluffs bordering the Desplaines River Valley on the south, and 

 there are similar bluffs on the north side of the valley at Lyons. 

 As a rule the strata of the limestone are approximately hori- 

 zontal, dipping slightly to the south. But at Stony Island they 

 are sharply inclined (Fig. 23) and that in opposite directions on 

 the two sides of the present low ridge, showing that this eleva- 

 tion is the eroded remnant of what was once a mountain fold of 

 no mean height. Similar folds of the strata on a smaller scale 

 are seen in many of the quarries. 



Below the Niagara limestone occurs a succession of rocks 

 which in a deep boring at Lockport were disclosed as shown in 

 the accompanying diagram (Fig. 24). Other similar borings 

 have given like results. Presumably if the drill had gone deeper it 

 would have encountered the still older rocks of the Proterozoic era 

 and then the complex of the Archaeozoic. The history of the for- 

 mation of these rocks of our immediate vicinity is most interesting. 



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