Geologic units in Rock county, Minnesota

Undifferentiated (Cenomanian to Campanian) at surface, covers 62 % 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 38 % 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).

Sioux Quartzite (Lower Proterozoic) at surface, covers < 0.1 % of this area

Pink and reddish to tan, siliceous, fine- to coarse-grained, iron-stained orthoquartzite with minor metamorphosed conglomerate and mudstone layers. Estimated thickness greater than 1,000 ft (305 m).

Cretaceous, undifferentiated (Upper to Lower Cretaceous) at surface, covers < 0.1 % of this area

Black opaline spiculite, gray to black shale, yellow-brown to gray chalk, gray silty clay, and pink quartz-rich sandstone. Includes the Split Rock Creek Formation and other near-shore facies of the Dakota Formation, Graneros Shale, Greenhorn Formation, Carlile Shale, Niobrara Formation, and Pierre Shale. Thickness up to 400 ft (122 m).

Corson Diabase (Lower Proterozoic) at surface, covers < 0.1 % of this area

Black, intermediate- to coarse-grained diabase composed of plagioclase, pyroxene, and olivine.