Examination of cable-tool cuttings from water wells has resulted in the establishment of four zones in the Plattin and two zones in the Joachim. The zoning is based on lithologic character and insoluble residues. A cross section through fourteen selected wells extending approximately 300 miles, from Kirksville in northeast Missouri to Cape Girardeau in southeast Missouri, indicates the position and thickness of the zones where present.

At Cape Girardeau the Plattin is 600 feet thick and the Joachim is 250 feet thick. In the type areas, approximately 75 miles northwest, the Plattin and the Joachim are each only 200 feet thick. In northeast Missouri,in the vicinity of Kirksville, both formations are overlapped by the "Decorah," where the latter rests on the St. Peter. The cross section indicates that older beds are overlapped from southeast to northwest and suggests an unconformity at the base of the "Decorah."

Some of the subsurface zones have been located at the outcrop of widely separated localities and can be used in surface mapping. Redefinition of the Plattin-Joachim boundary is desirable. The name Rock Levee, derived from a place of that name approximately 3 miles southwest of Cape Girardeau and near the intersection of Federal Highways 25 and 61, is proposed for the rocks lying between the redefined Plattin and Joachim. The term "Decorah" as used in Missouri requires more specific definition.

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