32 Trans. Acad. Sci of St. Louis. 



come filled to some extent bv sand and finer material at 

 places where previous to this glaciation, erosion had been 

 in progress." 



This is exactly what one would expect if an ice sheet 

 had occupied the valley for several miles near the Chain 

 of Rocks for a brief period. It seems impossible to ac- 

 count otherwise for granite bowlders h^ng in the bluff 

 seventy feet above the present flood plain. 



On account of its possible relation to the drift in St. 

 Louis, not in the Chain of Rocks region, it appears to 

 the writer that the drift, mentioned by Fenneman,-^ as oc- 

 curring two miles north of the pumping station at the 

 Chain of Rocks, merits a fuller description than has 

 hitherto been given. 



The stream bed above the bridge lies in drift. (See 

 Fig. 10.) In the bed above and below the bridge may 

 be seen bowlders of Sioux quartzite, weighing approxi- 

 mately 200 pounds each. The igneous material among 

 these bowlders is scarce, probably owing to its advanced 

 decomposition. In the sections exposed by the stream 

 there are igneous pebbles, but generally in the lower de- 

 posit they are too much weathered to stand removal with- 

 out shattering. It appears that the granites are more 

 weathered than the basalts. 



The section exposed near the bridge by the main 

 stream and a tributarv entering at right angles 

 gives a clue as to the nature of the ridge on the left 

 facing up stream. At the top one finds soil and loess, 

 overlying a light yellowish brown bowlder clay, which 

 lies on top of stratified clay almost compact enough to 

 be called shale. Below the latter is a reddish brown 

 drift, eight to ten feet thick. Up stream a short distance 

 one may find tlie bed of the creek in this deposit. (See 

 Fig. 10.) The bowlders taken from this lowest deposit 

 are chiefly Sioux quartzite, and close textured sandstone. 

 The crystallines are thoroughly weathered. In some cases 



"•N. M. Fenneman. Illinois Geol. Surv. Bull. 12:9. 1909. 



