# St. Peter Sandstone Canonical page: https://lithomap.app/resources/geologic-formations/united-states/st-peter-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 Mississippi Valley and interior basins Age: Middle Ordovician Age detail: Middle Ordovician, with regional variation in its precise stratigraphic limits Depositional setting: Shallow marine shoreface and reworked coastal sand Rock types: Quartz sandstone, Sandy dolostone, Shale States in recognized usage: Arkansas, Illinois, Iowa, Michigan, Minnesota, Missouri, Wisconsin ## Summary The St. Peter Sandstone is a notably quartz-rich Ordovician unit spread across the central United States, important as an aquifer, industrial-sand resource, and record of repeated marine reworking. ## Distribution The St. Peter crops out around structural highs and basin margins in the Upper Midwest and Ozark region and lies deep underground within the Illinois and Michigan basins. Its thickness and lower contact reflect an irregular older erosion surface. Clean, pale sandstone is characteristic, but shale, carbonate cement, and dolomitic intervals occur locally. Some older usage combines adjacent beds differently, so map-unit boundaries must be checked regionally. ## Geological history Long transport and repeated wave or wind reworking removed many unstable grains and rounded the surviving quartz. Marine transgression redistributed that sand across a broad surface, producing well-sorted beds and local cross-stratification. During burial, quartz, dolomite, calcite, anhydrite, illite, and potassium feldspar formed or dissolved in different basin zones. Those changes strongly affect porosity, groundwater flow, and industrial quality. ## Fossil record Body fossils are generally sparse because clean sand was an energetic and often mobile substrate. Trace fossils and rare conodont-bearing or shaly intervals provide more useful biological and age information. The absence of conspicuous fossils is itself characteristic of many exposures, but it should not be treated as proof of a specific formation without stratigraphic context. ## Minerals and geological resources Quartz commonly makes up most of the rock. Minor feldspar, clay minerals, glauconite, dolomite, calcite, anhydrite, iron oxides, and authigenic illite occur in some areas. High-purity sand has been quarried for glass and other industrial uses, and the formation is an important aquifer. Both values depend on local grain size, cement, depth, and water chemistry. ## Field context - Quarries are active industrial sites and require explicit permission. - Friable sandstone cliffs can shed blocks or collapse along undercut faces. - A clean quartz sandstone is not diagnostic by itself; confirm its position between mapped units. ## Connected field guides - Quartz (mineral; Named rock type and mineral context) — https://lithomap.app/resources/mineral-identification/quartz - Dolostone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/dolostone - Sandstone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/sandstone - Shale (rock; Named rock type) — https://lithomap.app/resources/rock-identification/shale - Calcite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/calcite - Dolomite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/dolomite 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 - St. Peter diagenesis in the Illinois Basin — U.S. Geological Survey: https://pubs.usgs.gov/publication/b2094A - St. Peter conodonts and stratigraphy — U.S. Geological Survey: https://pubs.usgs.gov/publication/70027904 - Cambrian–Ordovician aquifer system — U.S. Geological Survey: https://pubs.usgs.gov/ha/ha730/ch_j/J-text8.html ## Formation photograph - Image: https://lithomap.app/_delivery/formations/st-peter-sandstone.webp - Description: Pale St. Peter Sandstone bluff beneath Joachim Dolomite in Pacific, Missouri - Creator: Kbh3rd - License: Public domain — https://creativecommons.org/publicdomain/mark/1.0/ - Source record: https://commons.wikimedia.org/wiki/File:St_Peters_Sandstone_Pacific_MO_5-med.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.