RELATIVE ANTIQUITY OP GRAVELS AND DRIFT. 185 



South of the drift the gravel is often accompanied by considerable 

 layers of sand. This sand may be interlaminated with the gravel, par- 

 ticularly in its lower parts, and often forms its substratum. In like 

 manner in the counties referred to, far north of the southern boundary 

 of the drift, considerable beds of sand locally accompany the gravel and 

 sometimes remain where the gravel lias been entirely removed. 



Both the sand and the gravel have yielded of their substance to the 

 till which overlies them. So generous has been their contribution that 

 locally the drift is often largely composed of their materials. Where this 

 is the case deep sections frequently show a remnant of the sand and 

 gravel beneath the till in undisturbed position. From this relationship 

 it was at once suggested that the influence of these sands and gravels in 

 determining the character of the till over the region where they once 

 existed might be a means of helping to determine the former northern 

 extension of the gravels and sands. Acting upon this suggestion, the 

 area farther north was studied, and what are believed to be unmistakable 

 evidences of gravel corresponding to the formation of the south are found 

 in the drift as far north as Henderson county and probably as far north 

 as Rock Island county; but Rock Island county is not far from the 

 southern border of the northern main driftless area. 



Relations of the tre-Pleistocene Gravels. 



It will be observed from what has been said that this formation of 

 gravel regarded as pre-Pleistocene occurs south of the drift, extends north- 

 ward witb considerable interruptions to the border of the drift, reappears 

 in the driftless area of Calhoun and Pike counties, passes beneath the 

 drift north of this area, and may be recognized to the northward either 

 in positions subjacent to the drift or by its contribution to the drift well 

 toward the northern driftless area. 



Several years since, while studying the driftless tract of southwestern 

 Wisconsin, the writer had occasion to notice certain gravels which had 

 been earlier described, but which manifestly had nothing to do with 

 glacial drift. No satisfactory explanation of their origin had ever been 

 offered. Similar gravels occur a( certain localities in the driftless south- 

 easl corner of Minnesota. It is now believed to he possible and even 

 probable that these gravels in the northern driftless area are to be corre- 

 lated with those farther southward. If this he true the pre-Pleistocene 

 ( presumably Tertiary I gravels have ;i far greater northerly extension than 

 has heretofore been known ; and this remains true, though the extension 

 is less great, whether the gravels of the Wisconsin driftless area are cor- 

 related with the gravels of the south or not. 



