Fossils in the St. Peter sandstone series are valueless for exact stratigraphic determinations. In Minnesota and in the Ozark region, the beds overlying the St. Peter series were formerly correlated with the Stones River group, but are now known to be Black River in age. In both regions, strata of Beekmantown age underlie the St. Peter series. In the east, the Black River and Chazyan groups are present, but the St. Peter series is absent. In these three type outcrop regions, there is no evidence to support the generally accepted belief that the St. Peter-Buffalo River sandstone series is pre-Chazyan in age.

In the Arbuckle Mountains, Oklahoma, the Simpson group crops out as a thick series of formations which range from lower Chazyan to middle Mohawkian in age. Sediments entirely similar to the St. Peter series are present in the outcrops of this group and underground in Oklahoma. Chaos and controversies now rule Simpson stratigraphy.

When the surface beds have been mapped and studied in as great detail, and by the same stratigraphers using the same microscopic methods, as now applied to the samples of Ordovician rocks penetrated by wells drilled for petroleum, detailed and accurate correlations can be made between the surface strata and the buried rocks connecting separated outcrops; then, and only then, can the presence or absence and (if present) the stratigraphic position and exact age of the St. Peter series be determined and established.

Work to the present time indicates that when such studies have been completed the St. Peter series will be found in Oklahoma beneath beds of known Black River age, just as it occurs in the Upper Mississippi Valley, and above beds of known Chazyan age.