The Winn Formation of the Saginaw Group is here named in the subsurface of the Michigan basin. It consists of gray, soft, clayey shale, siltstone, and sandstone, and the Verne Member in the upper part, a name used for a persistent thin bed of calcareous black shale or argillaceous limestone containing brachiopods and mollusks, which occurs in the Pennsylvanian in Michigan. Gradationally overlies the Lake George Formation; unconformably underlies unnamed Jurassic red beds or unconsolidated post-Jurassic deposits. The lower part of the Winn represents channel deposits of fluvial origin, and the Verne Member represents a return to marine conditions. Thickness is 133 ft at type well. Age is Early and Middle Pennsylvanian (late Morrowan to late Desmoinesian).

Source: GNU records (USGS DDS-6; Reston GNULEX).