250 C. P. BERKEY PALEOGEOGRAPHY OF SAINT PETER TIME 



maximum advance margins are held near the existing known outcrops on 

 the thin edges of equivalent beds. 



Summary 



Such facts as are known about the Saint Peter sandstone have been 

 considered by the writer in support of the following general statements : 



The Saint Peter sandstone is a mechanical sediment. 



The sands were derived in large part from the upturned edges of pre- 

 ceding beds of sandstone and quartzites bordering the continental areas, 

 and from the Basal Sandstone formation in particular, under conditions 

 that made this an easy and abundant supply. The use of an already 

 assorted material is one step in the production of its unusual characters. 



The purity of the formation, the rounded and uniform size of grain, 

 and its wide distribution over such a flat area indicate some agency of 

 extraordinary sorting efficiency and transporting capacity. This is cred- 

 ited to the winds. The folded and faulted character locally of the under- 

 lying Shakopee with no effect on the overlying Stones River beds point 

 to dynamic disturbances that occupied the Saint Peter epoch and ceased 

 at its close. Erosional unconformity between the Saint Peter and the 

 Shakopee indicates that the same time was marked by a retreat of the 

 sea. The widespread formation with uniform characters suggests a 

 withdrawal of the sea from a very large area in the upper Mississippi 

 valley. 



As a consequence, there was continuous deposition in the south and 

 interrupted sedimentation in the north. The Saint Peter sandstone is 

 stratigraphically a wedge including within itself a wedge-like break. In 

 age the thin southern edge is younger than the lower beds and older than 

 the highest beds, as they appear in the northern areas. The Saint Peter 

 therefore should be found to overlap the Magnesian series seaward, but 

 in turn should be itself overlapped by the limestones and shales of the 

 Stones River group in its succeeding development landward. In certain 

 localities it may be possible to diyide_the_ formation into an upper and a 

 lower Saint Peter. 



