PLEISTOCENE DEPOSITS BENEATH TILL SHEET. 107 



PRE-ILLINOIAN (?) TILL AND ASSOCIATED DEPOSITS. 



As yet the evidence pointing toward the occurrence of drift of greater 

 age than the Illinoian in the districts to the east, which lie beyond the limits 

 of the Keewatin ice sheet, has not sufficient strength to make it seem 

 advisable to advocate a pre-Illinoian till. There are occasional well sections 

 reported to have passed through a bed of wood or of soil in the midst of 

 the till in central Illinois. Thus at Pana a section reported by Worthen is 



as follows: 1 



Section of well at Pana, Illinois. ^ 



11 



Soil and clay ^ 



Blue clay 12 



Sand and gravel ". lg 



Hard red clay g + 



Forest bed 57 " 



Blue clay 2i 



Black soil or forest bed " ig 



Blue clay 



127 



Total 



This section is based upon specimens preserved from a test boring with 

 diamond drill. The specimens were recently examined by the writer and 

 some light obtained concerning the interpretation to be put upon this sec- 

 tion. The upper 15 feet consists of a pebbleless material to be classified 

 with the loess. The upper forest bed proves to be simply fragments of 

 wood embedded in ordinary till. The till was found to be very calcareous 

 at the horizon of this wood and to show no evidence of atmospheric expo- 

 sure subsequent to its deposition. The wood appears, therefore, to be 

 simply material deposited in the till by the ice sheet and has no more signifi- 

 cance in determining a time interval than the presence of a Paleozoic fossil 

 embedded in the drift. The lower forest bed consists of a humus-stained 

 clay in which fragments of wood occur; it is apparently a soil. Under 

 it is a greenish clay subsoil, such as occurs beneath swamps. This, as 

 well as the soil, is pebbleless. At the bottom of the clay, resting on the 

 limestone rock, there is a thin bed of ferruginous conglomerate, m which 

 angular chert is mingled with waterworn pebbles. No Canadian rocks or 

 specimens which can be referred to glacial action were found. This raises 

 the question whether the conglomerate is n ot of preglacial age. The soil 



i Geol. Illinois, Vol. VII, pp. 22-23; also Vol. VIII, p. 15. 



