THE NEBRASKAN ICE INVASION 225 



of the life of this stage. Other records of life from the surrounding territory 

 are correlated with this stage, and it is believed that a biota large and varied 

 enough for comparison with earlier and later stages is now available. 



a. TYPICAL EXPOSURES OF DEPOSITS 



The most t}*pkal exposure of Aftonian gravel and sand occurs between 

 Afton and Thayer in Union County, Iowa, 4 where the deposits lie below Kansan 

 drift. Sections nearby show the Nebraskan resting on bed rock. No organic 

 remains were found here, but in Harrison and Monona Counties, evidences of 

 life are found in abundance. A topical section is given by Shimek as follows: 5 



5. Loess, appearing above the cut, and ascending to top of bluff 



4. Loveland, a reddish joint clay, with lines of very large calcareous nodules, more 



than 15 feet 



3. Kansan, typical bluish, very calcareous till 12 " 



2. Aftonian: 



Fine whitish silt, about 15 " 



Fine silt, mixed with sand, shell bearing 5 " 



Coarse gravel, very ferruginous, about 7 " 



This reaches 10 feet in the northernmost cut 

 Fine cross-bedded sand 6 to 12 " 



1. Nebraskan drift, exposed 10 feet, but running out both ways 



The cuts in which these sections are shown form an almost continuous 

 section over 500 feet in length; it is 25 feet above the Little Sioux River, about 

 half a mile south of the county line between Harrison and Monona counties. 



b. TYPICAL AFTONIAN FAUNA 



The typical Aftonian fauna as described by Shimek 6 and Calvin 7 is as 

 follows: 



MOIXUSCA 



Aquatic species 



Quadrula metancvra Ancylus rividaris 



Lampsilis anodontoides Galba reflexa 

 Sphaerium sulcatum " caperata 



Pisidium abditum " humilis modicella 



" coniprcssum Physa integral 



Amnicola species Planorbis antrosas (=bicarinalus) 



" emarginata parvus 



Valvata tricarinata dilatatus 



bicarinata Segmentina armigera 



* Bain, Proc. Iowa Acad. Sci., V, pp. 86-101, 1897. 

 4 Geol. Iowa, XX, p. 338. 



6 Op. cit., pp. 316-342; Hay, Iowa Geol. Surv., XXIII. The vertebrate names have been 

 made to conform to this work. 



'Bull. Geol. Soc. Amer., XX, pp. 341-356; XXII, pp. 207-216. 



