The St. Peter sandstone is the most widespread arenaceous terrane of the Ordovician in the Mississippi Valley region (Fig. 1). The formation has been studied and described in Illinois, Missouri, Arkansas, and Oklahoma, but has not been given a comprehensive sedimentary analysis in the upper Mississippi Valley. This study was undertaken in an attempt to determine the characteristics of the formation nearer the source of its sands and to compare the St. Peter of Minnesota and Wisconsin with that of the regions farther south (Fig. 1).

It has been stated frequently that the St. Peter sandstone was derived from the Potsdam sandstone that cropped out around the south margin of the pre-Cambrian shield in the upper Mississippi Valley region during Ordovician time. These formations, already composed of well-sorted and well-rounded components, supplied the sands that characterize the St. Peter sandstone. These sands were delivered to the Ordovician sea by . . .

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