1 76 ERA NK LE VERE TT 



As to their origin the Buchanan gravels are made up of materials 

 derived from the Kansan drift. As to age they must have been laid down in 

 a body of water immediately behind the retreating edge of the Kansan ice. 



Manifestly the deposition of the Buchanan gravels covers 

 but a small part of the time between the Kansan retreat and the 

 Iowan advance. Unless therefore the deposition and subsequent 

 weathering both be included under this name it does not fill an 

 interglacial stage. Were there no Illinoian glacial stage to 

 break the continuity of interglacial conditions from the Kansan 

 to the Iowan stage of glaciation it would not seem necessary to 

 look for other terms. But in view of this glacial interruption 

 there seems need for names which will stand for the weathered 

 zones above and below the Illinoian till sheet. It is for this rea- 

 son that the name Sangamon is here suggested for a weathered 

 zone separating the Illinoian till from the overlying loess. In 

 an accompanying paper the name Yarmouth is introduced for 

 the weathered zone between the Illinoian and Kansan till sheets. 

 The name Buchanan may still have the significance given it by 

 Professor Calvin ; and if weathering be included may perhaps 

 be used to cover the time involved in the two interglacial stages 

 with the intervening glacial stage. 



THE SANGAMON WEATHERED ZONE. 



Earliest recognition. — Apparently the first recognition of the 

 occurrence of a definite soil horizon between the Iowan loess 

 and the Illinoian till sheet is that reported by Professor A. H. 

 Worthen, in the Geology of Illinois. 1 In his report on Sanga- 

 mon county, Illinois, made in 1873, Professor Worthen called 

 attention to a soil found at the base of the loess in Sangamon 

 and neighboring counties. The soil apparently was first noted 

 by Mr. Joseph Mitchell, in the excavation of wells in the north- 

 west part of the county and in neighboring portions of Men- 

 ard county. Mr. Mitchell furnished for publication in the 

 Geology of Illinois the following section of the beds usually 

 penetrated. 



'Geol. of Illinois, Vol. V, 1873, pp. 306-319. 



