# Mount Simon Sandstone Canonical page: https://lithomap.app/resources/geologic-formations/united-states/mount-simon-sandstone Last reviewed: September 17, 2026 Page purpose: Comprehensive United States geologic formation reference for human readers, search systems, and language models. Major region: Midwest / Great Lakes Regional setting: Upper Midwest and central interior basins Age: Late Cambrian Age detail: Late Cambrian, resting on much older Precambrian basement Depositional setting: Basal coastal, fluvial, and locally eolian sand during early Paleozoic flooding Rock types: Quartz sandstone, Conglomeratic sandstone, Shaly sandstone States in recognized usage: Illinois, Indiana, Iowa, Kentucky, Michigan, Minnesota, Ohio, Wisconsin ## Summary The Mount Simon Sandstone is a thick basal Cambrian sandstone laid across ancient crystalline basement, serving today as a major deep aquifer and a target for subsurface fluid storage research. ## Distribution The unit is exposed around parts of Wisconsin and Minnesota but lies hundreds to thousands of feet underground across the Illinois and Michigan basins and neighboring states. It thins over basement highs and thickens toward basin centers. Its lower surface covers deeply weathered Precambrian rock, while the finer Eau Claire Formation usually lies above. Historical group assignments and the inclusion of local sandstone equivalents vary by state. ## Geological history Coarse sand and local gravel first accumulated across an eroded basement landscape, followed by finer coastal and shallow-marine sand as Cambrian seas advanced. Cross-bedding records migrating dunes, bars, or channels in different parts of the system. Burial compacted and cemented the sandstone but preserved substantial pore space in many areas. Water becomes increasingly saline toward deep basin interiors, where the unit is isolated from modern recharge. ## Fossil record Body fossils are rare in most of the coarse sandstone. Vertical Skolithos-like burrows and other trace fossils occur in some beds and indicate colonization of sandy substrates. Sparse fossils make regional position and well-log character more important than paleontology for identification. Fossil absence should not be interpreted as a lifeless Cambrian environment. ## Minerals and geological resources Quartz is dominant, with feldspar, clay minerals, iron oxides, and local carbonate cement. Basal beds may contain pebbles derived from underlying granite and other Precambrian rocks. The Mount Simon is a groundwater aquifer and a studied reservoir for carbon dioxide storage. Suitability depends on porosity, permeability, caprock integrity, depth, and brine chemistry—not merely the formation name. ## Field context - Surface exposures represent only a small, shallow part of a much larger subsurface unit. - Loose or undercut sandstone faces can collapse without warning. - Deep-well water may be saline and is not equivalent to water quality near recharge areas. ## Connected field guides - Quartz (mineral; Named rock type and mineral context) — https://lithomap.app/resources/mineral-identification/quartz - Sandstone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/sandstone - Feldspar (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/feldspar - Granite (rock; Mineral discussed in this formation) — https://lithomap.app/resources/rock-identification/granite Mapped location and formation context support an identification but cannot establish one. Compare several physical properties and use laboratory analysis when the material remains ambiguous. ## Connected state geology guides - Michigan geology — https://lithomap.app/resources/geology/united-states/michigan ## Authoritative sources - Geolex Mount Simon record — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/Units/MountSimon_2887.html - Principal U.S. frac-sand source units — U.S. Geological Survey: https://pubs.usgs.gov/of/2015/1107/pdf/ofr20151107.pdf - Northern Midwest aquifer hydrogeology — U.S. Geological Survey: https://www.usgs.gov/publications/hydrogeology-cambrian-ordovician-aquifer-system-northern-midwest-united-states-a ## Formation photograph - Image: https://lithomap.app/_delivery/formations/mount-simon-sandstone.webp - Description: Iron-stained Mount Simon Sandstone displayed in a deep Warren County drill core from Ohio - Creator: James St. John - License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ - Source record: https://commons.wikimedia.org/wiki/File:Mt._Simon_Sandstone_(Middle_Cambrian%3B_Warren_County_core%2C_Ohio%2C_USA)_1.jpg Formation-level information does not prove that a fossil or mineral occurs at a specific coordinate. It also does not establish ownership, access, collecting permission, present conditions, or specimen identity.