EIDGED DRIFT OF KASKASKIA BASIN. 71 



scarcely one-half mile out from the margin of the Illinoian drift, The 

 presence of the soil beneath this gravel makes it evident that there was but 

 little erosion accompanying the deposition of the gravel. It also shows that 

 the gravel is not a residue of coarse material formed in the process of ero- 

 sion of the Kansan till sheet, either before or after the Illinoian glaciation. 



The principal valley affording a line of drainage for the ice sheet was 

 the Mississippi, only short sections of the eastern tributaries being outside 

 the Illinoian drift margin. This valley was covered by the ice sheet, as 

 already indicated, from near the south end of the Driftless Area down to 

 the vicinity of Fort Madison, Iowa. Possibly also it was encroached upon 

 for a few miles in the vicinity of St. Louis, Missouri. The blockade in the 

 portion bordering eastern Iowa was so complete as to cause the opening of 

 a temporary line of drainage across eastern Iowa outside the ice margin, as 

 indicated below; but there appears to have been at most only a partial 

 blockade near St. Louis. An examination into the character of the deposits 

 in the Mississippi Valley, between Fort Madison and St. Louis, has brought 

 to light nothing to indicate vigorous drainage at the Illinoian stage of glaci- 

 ation. Indeed, the valley seems to have become filled to some extent by 

 sand and finer material at places where, previous to this glaciation, erosion 

 had been in progress. This is markedly the case just below the lower 

 rapids. The filling there is mainly silt and fine sand, though a fine gravel 

 appears in places where the cm-rent was strongest. This matter is discussed 

 more fully below (pp. 94-96). In the section of the Mississippi also between 

 St. Louis and Cairo only sand and silt are found along the valley, a feature 

 that apparently indicates a drainage no more vigorous than at the present 

 day. Yet it seems probable that at times the volume of water greatly 

 exceeded that now discharged through the valley. 



Much remains to be learned concerning the drainage conditions attend- 

 ing this and later stages of glaciation, but from what is now known the 

 drainage at the Illinoian stage appears to have been very sluggish not 

 only on the Mississippi, but on all the valleys leading away from the ice 

 sheet. 



THE EIDGED DRIFT OF THE KASKASKIA BASIX. 



The position of the principal ridges in this system may be seen by 

 reference to the glacial map (PI. VI). It will be observed that there is one 

 belt lying near the Kaskaskia River and found chiefly on its west border. 



