Several of the deeper wells in the Central Basin and Highland Rim areas of Tennessee have penetrated sandy zones in the Lower Ordovician at depths ranging from 1,250 to 1,600 feet below the Chattanooga shale and from approximately 600 to 950 feet below the base of the Trenton limestones. These zones, which average less than 10 feet in thickness, are composed of subangular to well rounded, glassy, and frosted quartz grains associated with white to gray, granular, and commonly cherty, magnesian limestone and dolomite. This unit has been correlated by some geologists with the St. Peter sandstone of the Mississippi Valley region.

Recent subsurface studies, especially by the insoluble-residue method, have allowed zoning of the pre-Stones River rocks of middle Tennessee. Examination of samples from 30 wells demonstrates that where present the so-called "St. Peter" occurs below the top of the Knox dolomite group of upper Canadian age. These occurrences with relation to established subdivisions of the Knox dolomite group show that the "St. Peter" is not a definite stratigraphic zone although it generally occurs within the upper 300 feet of the Canadian.