THE KANSAN ICE INVASION 249 



Brown till, calcareous, fracturing in cubic blocks, color changing to grayish blue 



at 12-15 feet; a characteristic Kansan till 25 feet 



Height of section about.. 75 feet 



In Lee County, in the bluffs near Fort Madison, the Kansan till contains 

 vegetation among which Penhallow identified: 10 



Larix americana? 

 Taxus canadensis? 

 Taxus species 



The vegetation is said by Keyes to be embeded in the blue till, but no depth 

 is stated. As the Kansan is here 60 feet in thickness, and is overlaid by yellow- 

 ish Illinoian drift, it is assumed that the organic remains were in the upper part 

 of the Kansan drift, and hence of Yarmouth age. In the same county, 11 

 "between Fort Madison and Montrose there are high bottoms, consisting of 

 beds of sand and gravel, the surface of which is from 20 to 30 feet above the 

 present high water mark. Along the rapids there is a bed of Unios in band-like 

 form, extending on both sides of the river at an elevation from 15 to 20 feet 

 above present high water mark, nearly the whole distance from Nauvoo to 

 Keokuk. Just below Mansion House at Nauvoo, the mussel bed is 25 feet 

 above ordinary level of the river. The shells are white and waterworn, the 

 deposit 12 to 18 inches thick. " The Unios mentioned have not been identified. 

 Like the benches along the Missouri River, these high bottoms are probably 

 remnants of the old Kansan drift plain and hence the Unios may be referred to 

 the Yarmouth Interglacial interval. 



Along the rivers in Plymouth, Harrison, and Monona counties, extensive 

 benches occur which have been mistaken for river terraces. These are said by 

 Shimek 12 to be remnants of the old drift plain showing the same structure as 

 the Kansan drift. "Near the mouth of Broken Kettle Creek, Plymouth 

 County, there is a higher terrace very faintly cut in the loess. Its upper sur- 

 face is marked by the presence of Unios 45 feet above low water in the Sioux. 

 This level corresponds to the flat and well-marked bottom land found in the 

 upper portion of Broken Kettle Creek. The Unios occur in little groups or 

 colonies, nested together along this horizon, and at the same horizon ordinary 

 loess fossils occur. The same phenomena are found in Woodbury County, at 

 North Riverside, though at a lower horizon relative to the present water level. 

 In Monona County, near Castana and Turin, there is a very well marked loess 



" Keyes, Geol. Surv. Iowa, III, pp. 357-358. 

 11 Worthen, Geol. Surv. Iowa, I, p. 186, 1858. 

 « Geol. Iowa, XX, p. 291. 



