UNCONFORMITY 



241 



conformity as a rule obtains at the greater distance from the continental 

 land margin of ancient Laurentia. In Missouri, according to Winslow, 

 there is an unconformity below the saccharoidal sandstone, now correlated 

 with Saint Peter, and erosion valleys have been described there.* 



Throughout the greater part of the region of the Saint Peter the lower 

 contact is not within surface observation, so that, even if unconformity 

 were general, most descriptions would necessarily fail to note it. 



In a few isolated areas where the original landward margins are still 

 preserved, or where ancient islands project through, the Saint Peter 

 seems to merge into the Basal sandstone, Potsdam, either by thinning 

 out of the Magnesian series or by its destruction and overlap, so that the 

 break is swallowed up in the uniform textured sandstone formation. 



Figure 4. — Stratigraphic Range of the basal Sandstone. 



This generalized sketch indicates the relationships of the Saint Peter formation to the 

 larger groups representing deep-sea deposits, and illustrating the conception of an erosion 

 Interval in Saint Peter time. 



This relationship is well shown and fully discussed in the Geological 

 Survey reports of Wisconsin. 



When the evidence of unconformity is lacking there may nevertheless 

 have been dry land at so nearly baselevel position that erosional forces 

 were largely spent upon the loose mantle of shifting sands left by the 

 outwash of the retreated sea. Here and there conditions were such as 

 to permit penetration of this mantle and imprint the marks of an erosion 

 interval on the undulating Shakopee. It is the writer's opinion that the 

 Saint Peter sandstone is of such composition as to permit obliteration 

 within itself of all marks of such an interval. This is probably true of 

 all coarse grained or very pure sandstone formations. Great interbedded 

 sandstones as a type doubtless represent periods of marked oscillations of 



Missouri Geological Survey, vol. vi, p. 357. 



