Geologic units in Freeborn county, Minnesota

Middle Devonian (Middle Devonian) at surface, covers 38 % of this area

Dolostone, sandy dolostone, limestone, and shale; includes the Chickasaw Shale and Bassett Member of the Little Cedar Formation, and Pinicon Ridge and Spillville Formations.

Upper Ordovician (Upper Ordovician) at surface, covers 38 % of this area

Limestone, shaley limestone, and dolostone; includes the Maquoketa Formation, and the Stewartville, Prosser, and Cummingsville Formations of the Galena Group. Also includes limestone and dolostone of the Red River Formation, and sandstone and shale of the Winnipeg Formation in northwest Minnesota.

Middle and Upper Devonian (Middle and Upper Devonian) at surface, covers 23 % of this area

Dolostone, limestone, and shale; Includes the Lithograph City Formation, Coralville Formation, and Hinkle and Eagle Center Members of the Little Cedar Formation.

Middle and Upper Ordovician (Middle and Upper Ordovician) at surface, covers < 0.1 % of this area

Shale, dolomitic limestone, and sandstone; includes the Decorah Shale of the Galena Group, the Platteville and Glenwood Formations, and the St. Peter Sandstone.