# Moonstone Formation

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The **Moonstone Formation** is a geological formation of late [Cenozoic](/source/Cenozoic) age, consisting of approximately 400 meters of [fluvial](/source/Fluvial) (river-related) and [lacustrine](/source/Lacustrine) (lake-related) deposits. It is best known for mantling the [Precambrian](/source/Precambrian) granites of the [Sweetwater Arch](/source/Sweetwater_Arch) in central [Wyoming](/source/Wyoming). The formation is significant for its rich vertebrate fossil record and its role in dating the subsidence and faulting that shaped the modern Wyoming landscape.

## Stratigraphy and Age

The Moonstone Formation unconformably overlies the lower to middle Miocene [Split Rock Formation](/source/Split_Rock_Formation). While early studies in the 1960s tentatively assigned it a Pliocene age, modern radiometric dating and mammalian biochronology have refined its placement to the late Miocene (late Barstovian to early Hemphillian ages). U-Pb zircon dating of interbedded volcanic ashes within the formation has yielded precise ages of approximately 8.39 to .24 Ma. This indicates that the formation was deposited between roughly 11.4 and 6 million years ago. The formation occurred as the [Seetwater Arch](/source/Seetwater_Arch) subsided, forming an east to west [syncline](/source/Syncline). Faulting in the region created the nearby [Seminoe](/source/Seminoe_Mountains) and [Ferris Mountains](/source/Ferris_Mountain) to the south.[1]

## Lithology

The Moonstone formation is composed of sedimentary and volcanic rocks, including sandstone, shale, claystone, limestone, volcanic ash, and chaledony.[2][3]

## Paleontology

Various fossils have been found in the formation. Some of those include extinct: [sabertooth cats](/source/Machairodontinae), [cricetid](/source/Cricetid) rodents, rabbits, and beaver. Black shale in the formation contains pollen and megaspores, indicating its age in the late [Clarendonian](/source/Clarendonian) period.[4]

## See also

- [Split Rock](/source/Split_Rock_(Wyoming)), which has evidence of the formation.

## References

1. ["Vertebrate biostratigraphy and U-Pb zircon geochronology of the Moonstone Formation"](https://gsa.confex.com/gsa/2002AM/webprogram/Paper42019.html). Geological Society of America. Retrieved 2026-01-18.

1. ["Geology of the Rocks of the Granite Mountains, Wyoming"](https://pubs.usgs.gov/bul/1121i/report.pdf). USGS Publications Warehouse. Retrieved 2026-01-18.

1. ["Cenozoic Geology of the Granite Mountains Area, Central Wyoming"](https://digital.library.unt.edu/ark:/67531/metadc1115055/). University of North Texas Digital Library. Retrieved 2026-01-18.

1. ["Magnetic stratigraphy and vertebrate biostratigraphy of the Moonstone Formation"](https://gsa.confex.com/gsa/2005AM/webprogram/Paper93032.html). Geological Society of America. Retrieved 2026-01-18.

## External links

- [Cenozoic Geology of the Granite Mountains Area, Central Wyoming (USGS Bulletin 1121-I)](https://pubs.usgs.gov/bul/1121i/report.pdf)
- [National Geologic Map Database – Moonstone Formation Details](https://ngmdb.usgs.gov/Geolex/UnitRefs/MoonstoneRefs_8519.html)
- [USGS Professional Paper 495-C: Cenozoic Geology of the Granite Mountains Area](https://digital.library.unt.edu/ark:/67531/metadc1115055/)

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Adapted from the Wikipedia article [Moonstone Formation](https://en.wikipedia.org/wiki/Moonstone_Formation) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Moonstone_Formation?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.