232 C. P. BERKEY PALEOGEOGRAPHY OF SAINT PETER TIME 



From the distribution of the outcrops/ together with the records of 

 deep wells and local stratigraphic interpretation, it is certain that the 

 Sain Peter area includes almost the whole of Iowa, Illinois, and Missouri, 

 together with large areas in Minnesota, Wisconsin, Michigan, and In- 

 diana, as well as less clearly defined portions of Nebraska, Kansas, Arkan- 

 sas, Indian Territory, and possibly North Dakota. The northerly border 

 of this great tract exhibits the upturned eroded edge of the Saint Peter 

 in characteristic fades. The southerly and easterly border, on the other 

 hand, passes more and more deeply beneath later sediments, beyond ob- 

 servation except by deep well records. These indicate, however, a grad- 

 ual increase of argillaceous and calcareous matter in this direction. The 

 distribution and boundaries are plotted on the accompanying outline map. 



General stratigraphic Position 



The Saint Peter is Ordovicic in age. In its northerly exposures it is 

 overlain by a limestone that is correlated with the Stones Kiver formation 

 by Ulrich and Winchell,* and it is underlain by a dolomite, called the 

 Shakopee, also Ordovicic in age, but of doubtful equivalence. The de- 

 scending series of dolomites, shales, and sandstones beginning with the 

 Shakopee, known as the ''Magnesian series," f carries somewhere the line 

 dividing Cambric from Ordovicic time, but the exact position of it is 

 uncertain. The base of the Paleozoic column is represented by a thick 

 sandstone and conglomerate, commonly referred to as the Potsdam or the 

 Basal sandstone, or the Saint Croix formation, by workers in different 

 fields. 



The Saint Peter therefore occupies a position well toward the base 

 of the Ordovicic series, and it is the uppermost one of the five sandstone 

 formations which give a predominant arenaceous character to the basal 

 Paleozoic rocks of the northern Mississippi Valley region. This relation- 

 ship is indicated in figure 2. 



In other areas, especially southward, there may be conditions of over- 

 lap that modify minor stratigraphic relationships of succession, as will 

 be indicated in another paragraph. 



It has been common usage to correlate the Saint Peter with the 

 Calciferous (Beekmantown) or with the Chazy of New York. This has 

 been done more on the order of succession and similarity of rock type 

 than any direct evidence. The few fossil forms found give little help in 

 exact correlation. 



* Geol. and Nat. Hist. Survey of Minnesota, Final Report, vol. iii, part ii. pp. xciii-xciv. 

 t Bull. Geol. Soc. Am., vol. 6, pp. 167-198. 



