Geologic units in Martin county, Minnesota

Middle and Upper Cambrian (Middle and Upper Cambrian) at surface, covers 34 % of this area

Sandstone, siltstone, and shale; includes the Wonewoc Sandstone, Eau Claire Formation, and Mt. Simon Sandstone.

Upper Cambrian (Upper Cambrian) at surface, covers 19 % of this area

Sandstone, siltstone, shale, and dolostone; includes the Jordan Sandstone and St. Lawrence Formation, and the Mazomanie, Lone Rock, and Davis Formations of the Tunnel City Group.

Lower Ordovician (Lower Ordovician) at surface, covers 17 % of this area

Dolostone, sandy to silty dolostone, and sandstone; includes the Shakopee Formation and Oneota Dolomite of the Prairie du Chien Group.

Middle and Upper Ordovician (Middle and Upper Ordovician) at surface, covers 10 % 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.

Undifferentiated (Cenomanian to Campanian) at surface, covers 7 % of this area

Conglomerate, sandstone, mudstone, shale, marlstone, siltstone, and minor lignite, deposited in marine and non-marine settings; likely Cenomanian to Campanian age. Unit outline is the product of contouring the stratigraphic top and base, from which an isopach grid was created. Because the distribution is patchy, unit boundaries were drawn from the gridded data to represent locations where more than 25 feet (8 meters) of thickness occurs. As a result, many areas outside of the unit boundaries may be overlain by thin Cretaceous strata and the unit is depicted without a contact line.

Sioux Quartzite (Paleoproterozoic) at surface, covers 6 % of this area

Quartzite, mudstone, and local conglomerate of fluvial and marine origin (~1,760 to 1,630 Ma; 1,902 ± 55 Ma rhyolite pebble in basal conglomerate).

Upper Ordovician (Upper Ordovician) at surface, covers 5 % 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.

Granitoid gneiss with dioritic to amphibolitic enclaves (Mesoarchean to Paleoarchean) at surface, covers 0.4 % of this area

Granitoid gneiss with dioritic to amphibolitic enclaves. Produces moderately high and varied gravity and magnetic signatures.

Granitic rocks, undifferentiated (Paleoproterozoic) at surface, covers 0.4 % of this area

Granitic rocks, undifferentiated. Largely inferred from geophysical maps.