IOWA ACADEMY OF SCIENCES. 73 



tion of the margin of the Illinoian drift in any of these counties. 

 It is also not fully decided whether it reaches to the border of 

 the driftless area in Jo Daviess and northwestern Carroll coun- 

 ties, 111. , and in southwestern Wisconsin. The balance of proba- 

 bilities, however, seem to favor its extending to the Driftless 

 area. 



The Illinoian till sheet overlaps, a few miles, the Kansan till 

 sheet of the western lobe from the latitude of Hannibal, Mo., 

 northward to the vicinity of the southern point of the Driftless 

 area. In this region of overlap a weathered zone is developed 

 between the Illinoian and Kansan till sheets at the level of the 

 outlying Kansan surface as indicated below. 



Introduction of the name Illinoian. — The tracing of this south- 

 western border of the Illinois lobe was begun by the writer in 

 the autumn of 1892 and carried as far north as Hancock county, 

 Illinois, that season No opportunity to continue the study 

 was offered until the spring of 1894, when the mapping of the 

 border was carried from Lee county, Iowa, northward to Scott 

 county. The greater part of the data presented in this paper, 

 and conclusive evidence of a long interval between the deposi- 

 tion of the till sheets now known as the Kansan and Illinoian, 

 and also the evidence that the Illinoian is much older than the 

 lowan had been obtained as early as June, 1894. The writer 

 then began to use the name Illinoian in correspondence, but it 

 seemed best to defer its introduction into literature until oppor- 

 tunity had been afforded other geologists to examine it. In 

 August, 1896, Prof. T. C. Chamberlin and Mr. H. F. Bain 

 were conducted by the writer to some of the exposures in 

 southeastern Iowa, which show the soil above and below the 

 sheet formed by the Illinois lobe and each recognized the need 

 for a distinctive name for this drift sheet. The name was 

 accordingly soon introduced into geological literature by Pro 

 fessor Chamberlin (1). 



Other Interpretations. — At the ninth annual meeting of this 

 academy, held in December, 1894, Mr. F. M. Fultz read a paper 

 (2) in which the interpretation was presented that the ice lobes 

 alternated in the occupancy of the district south of the Driftless 

 area and that the latest occupancy was by the western lobe. 

 The extension of the eastern lobe into Iowa had been inferred 

 by him through the discovery of a bowlder of red jasper con- 



(1; See editorial, Journal of Geology, October-November, 1896, pp. 872-876. 

 (3) Procaedlngs Iowa Academy of Sciences, Vol. II, 1895, pp. 209-212. 



