Restudy of the Middle Ordovician Plattin formation of southeast Missouri to determine the mode of its northward thinning has led to the establishment of the Plattin group of four formations. The group consists of the following units, from older to younger.

1. Bloomsdale formation--oolitic and carbonate pebble-bearing calcilutite, commonly with green shale layers and beds of dolomite rock

2. Beckett formation--fucoidal fine-textured calcite limestone with many layers of intraformational conglomerate in the upper member

3. Hager formation--mainly of fine-textured calcite rock which grades northward into calcarenite and calcilutite

4. Macy formation--fucoidal calcite rock and succeeding fossiliferous calcite rock, the latter with shaly partings and a widespread metabentonite near the top

Northward thinning of the Plattin group, from approximately 400 feet at Cape Girardeau to approximately 125 feet in southern Lincoln County, is by convergence within units and offlap and overlap of units. The Beckett formation overlaps the oolite-bearing Bloomsdale. The Hager formation disconformably offlaps the Beckett. The Macy formation thins by the reduction of the fucoidal part; the upper member is essentially plate-like.

The Plattin has been classified as Bolarian, Black River in age, and correlated with the Platteville of the upper Mississippi valley. Faunules from the upper Macy suggest that those beds are lower Trentonian. Lower formations resemble the Bolarian in central Kentucky and central Pennsylvania.