POSTGLACIAL OXIDATION 289 



a place as any to illustrate this point is in the area between the Illinois 

 and Mississippi rivers extending from Galesburg northeast along the line 

 of the Burlington and Missouri Railroad. For a long distance this is a 

 tableland from 300 to 350 feet above the rivers mentioned on either 

 side. The glacial deposits over this area consist of a pretty uniform 

 capping of loess, 10 or 15 feet in thickness, overlying till of about the 

 same thickness on the average, but deepening to a hundred feet or more 

 in the depressions of the old valleys. But in descending on either side 

 one encounters ever-deepening preglacial valleys heading in character- 

 istic amphitheaters in which the evident erosion of the present streams 

 has had comparatively little influence in molding the surface. 



Postglacial Oxidation 



The extent of the oxidation of postglacial deposits has been generally 

 taken as one of the surest measures of the time that has elapsed since they 

 were laid down. There is no question about the increase of this oxida- 

 tion as we go southward to the glacial boundary, and also as in many 

 places we penetrate older 'glacial deposits that have been overridden by 

 later ones a considerable distance north of the extreme limit of the ice 

 extension. Xorth of the watershed in Ohio there is a pretty uniform 

 blanket of yellow till from 10 to 15 feet in thickness overlying a less 

 oxidized blue till of much greater average thickness. Moreover, between 

 these strata there is quite likely to be a deposit of gravel, indicating the 

 temporary influence of flowing water. Toward the southern part of the 

 State the thickness of this blanket increases until when we pass the 

 boundary of the so-called Wisconsin deposits the entire mass of till seems 

 to be thoroughly oxidized. 



But Prof. E. H. Williams again, in his extensive observations on the 

 attenuated border of glacial deposits in Pennsylvania, has brought to 

 light facts which have apparently escaped the notice of other observers, 

 and which lead to entirely dift'erent conclusions from those which have 

 ordinarily been made respecting the age of the so-called Illinoisau, Kau- 

 san, and pre-Kansan deposits. The facts are that, mingled with this 

 highly oxidjzed material throughout the "attenuated border" in Penn- 

 sylvania, there are pebbles of the same character which are only slightly 

 oxidized, indicating that the mass of oxidized material was already oxi- 

 dized when it was picked up and deposited by the ice, and is therefore 

 no criterion of the length of time which has elapsed since its deposition. 

 But it is the comparatively unoxidized material mixed with it which 

 indicates the time of the deposit. Furthermore, Mr. Williams observed 



