COMPARISON WITH THE KANSAN 115 



ering, however, descends along joints in the blue cla}' to a depth of 20 or 

 30 feet. 3. The lime constituent of the Kansan till was completel}^ re- 

 moved from the weathered zone, partly by leaching and probably in part 

 by the growth of vegetation during the longinterglacial intervals. That 

 leaching was the process most effectual in the removal of the lime carbo- 

 nate is indicated by a zone, from 8 to 12 feet below the surface, charged 

 with calcareous concretions resembling loess-kindchen. These concre- 

 tions evidentl}^ represent, in concentrated condition, the lime carbonate 

 originally distributed through the overlying portion of the till. The 

 material composing them was doubtless carried in solution by descend- 

 ing waters which were probably charged with carbon dioxide from de- 

 caying vegetation. Below the zone of leaching the blue Kansan till is 

 decidedl}^ calcareous. 4. Finally in the weathered zone certain kinds 

 of granites, as well as boulders of some other rock species, are completely 

 softened and decayed. 



That the erosion, oxidation, leaching, and rock decay enumerated 

 above took place before the advent of the lowan ice is clearl}^ indicated 

 by numerous facts. In the first place, there is convincing evidence re- 

 specting all the characters named beneath the lowan drift, but the deposi- 

 tion of the lowan covered up and protected the Kansan surface and put 

 an end to all the processes that worked for change. There has been no 

 change since. The Kansan surface is in the precise condition it was 

 when the lowan was laid down on it. In the second place, an examina- 

 tion of the belt, 4 or 5 miles wide, just outside the lowan margin, dis- 

 closes a very large number of satisfactory sections supporting the same 

 conclusion. When the lowan ice was at its maximum a heavy body of 

 loess was laid down within this belt, and indeed for many miles beyond 

 it. The loess was thick enough to protect the Kansan surface from the 

 effects of the atmosphere, as did the lowan till in the intra-marginal 

 area. Since the settlement and cultivation of the country rainwash has 

 cut deep trenches in the fields and along the roadsides, and so has re- 

 vealed in hundreds of instances the significant fact that the loess was 

 moulded over an eroded, oxidized, and leached surface of Kansan till, 

 the upper zone of which was characterized by the presence of numerous 

 boulders so far decayed that they fall to pieces by their own weight. On 

 the other hand, the lowan surface is not eroded to any measurable extent, 

 the surface presents no signs of oxidation or leaching, and the granitic 

 boulders show practically no indications of decay. Measured by the 

 effects produced the interval between the Kansan retreat and the lowan 

 invasion was many times the length of all post-Iowan time. Taking 

 account of all the facts, it would not seem extravagant to say that it was 

 50 times as long. It would certainly be very conservative to estimate it 



XVII— Bum,. Geol. Soc. Am.. Vol. 10, 1898 



