250 AMADEUS W. GRABAU 
in central Ohio; while the highest beds rest on Monroan or even 
Niagaran, in southern Ohio. Continuing southeastward, the black 
shale rises in the series, until in eastern Tennessee it is of Lower 
Mississippic age, and resis on Lower Siluric or on Ordovicic strata.’ 
(Pig 12)e 
DISCUSSION 
Professor Calvin 
I have studied the Saint Peter sandstone in Iowa, Wisconsin, Minnesota, and 
Illinois, and nowhere have I seen any marked indications of cross-bedding such 
as would be consistent with an aeolian origin of the formation. In Iowa and 
Minnesota there are few structural bedding planes seen in fresh sections, but those 
that do exist are always horizontal and parallel. Bedding planes are more numer- 
ous in this sandstone west of Ottawa, Ill., but they are all precisely of the character 
one sees everywhere in aqueous sediments. When the Saint Peter is exposed on 
sloping hillsides, by a process akin to exfoliation, it breaks off in thick flakes 
parallel to the exposed surface and so often presents a false appearance of cross- 
bedding; but this feature has no relation to the original structure. One hardly 
needs to go to the Libyan desert to ascertain the characteristics of aeolian sands. 
The region around the south end of Lake Michigan affords ample opportunity, 
nearer home, to study the structural features and topographic forms of wind- 
blown deposits. I have seen nothing in the Saint Peter suggesting similar origin. 
Furthermore, the Saint Peter occasionally contains marine fossils, as shown by 
Winchell and Sardeson. 
«See Grabau, “‘Types of Sedimentary Overlap,” Bull. Geol. Soc. Amer., Vol. 
XVII, pp. 593-613. 
