STRUCTURE OF THE DRIFT BORDER. 43 



The peaty deposits are usually found associated with sandy beds, while the 

 soil and weathered zone cap a sheet of till or stiff clay. 



In many of the wells in southeastern Iowa, and also in natural expo- 

 sures, a reddened or deeply oxidized clay is found, instead of peat or muck, 

 between the Illinoian and Kansan sheets. This constitutes usually the 

 weathered surface of the Kansan, and appears to have been originally a 

 calcareous till, like the remainder of that deposit, Acid tests have fre- 

 quently been made in freshly dug wells and in natural exposures with a 

 view to determine the amount of leaching prior to the deposition of the 

 Illinoian till sheet. It is found that, as a rule, no response to acid is obtained 

 at less than 4 feet, and often the response does not set in within 6 feet of the 

 top of the buried Kansan sheet. In the cases where leaching occurs within 

 4 feet of the surface, it seems safe to infer that a portion of the leached 

 material had been removed prior to the deposition of the Illinoian till sheet. 

 While the leached material usually bears a striking resemblance to the 

 underlying calcareous till, exposures have been found in which it differs in 

 general appearance and is perhaps a deposit of different origin. These 

 deposits also are thoroughly leached at surface. This weathered zone is so 

 conspicuous throughout the region of overlap of the Illinoian upon the Kan- 

 san that the writer has satisfied himself of the occurrence of a long interval 

 of deglaciation prior to the deposition of the Illinoian drift. Instances of 

 the occurrence of this weathered zone, and also evidences of erosion between 

 the Kansan and Illinoian glaciations, are presented below. 



With this brief explanation of the drift border in the part most familiar 

 to the writer the discussion will pass to the north part of the border in 

 southern Wisconsin, and proceed thence southward. 



From the descriptions of the drift in southern Wisconsin, presented 

 in the Geology of Wisconsin and in Chamberlin and Salisbury's paper in 

 the Sixth Annual Report of this Survey, it appears that the drift border is 

 characterized by occasional gravelly knolls and ridges, some of which are 

 of distinct esker type. There are also gravel and sand deposits on plane- 

 surfaced tracts, but the greater portion of the drift appears to be a moderately 

 stony till with an adhesive clayey matrix. This phase of the drift border 

 continues southward across northwestern Illinois. It is found that some of 

 the preglacial valleys near the drift border were filled with a deposit of fine 

 silt or clay containing very few pebbles. These deposits characterize 



