Geologic units in Madison county, Missouri

Eminence Dolomite, Potosi Dolomite (Late Cambrian-Croixian Series) at surface, covers 52 % of this area

Eminence Dolomite - dolomite with some druse - coated chert. Potosi Dolomite - dolomite with abundance of druse - coated chert.

St. Francois mountains volcanic supergroup (Precambrian Y) at surface, covers 18 % of this area

Chiefly alkali rhyolitic ash - flow tuffs, lava flows, and bedded tuffs, with minor trachytes.

Elvins Group - (including Derby-Doerun Dolomite, Davis Formation), Bonneterre Dolomite (Late Cambrian-Croixian Series) at surface, covers 12 % of this area

Derby-Doerun Dolomite - alternating thin dolomite, siltstone, and shale; Davis Formation - glauconitic shale with fine - grained sandstone, limestone, and dolomite); Bonneterre Dolomite - dolomite, dolomitic limestone, and limestone; glauconitic in lower part.

St. Francois Mountains intrusive suite (subvolcanic, alkali granitic ring complexes) (Precambrian Y) at surface, covers 9 % of this area

St. Francois Mountains intrusive suite (subvolcanic, alkali granitic ring complexes).

Lamotte Sandstone (Late Cambrian-Croixian Series) at surface, covers 5 % of this area

Sandstone with some dolomitic and shaly lenses; coarse - graines to conglomeratic and arkosic at base.

Gasconade Dolomite (Early Ordovician-Ibexian Series) at surface, covers 3 % of this area

Coarse - crystalline cherty dolomite with a basal Gunter Sandstone Member.

Diabase dikes and sills (Precambrian Y) at surface, covers 0.3 % of this area

Diabase dikes and sills.