The Renton Formation is a

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

Washington (state) Washington (), officially the State of Washington, is a state in the Pacific Northwest region of the Western United States. Named for George Washington—the first U.S. president—the state was formed from the western part of the Washin ...

within the

Puget Group The Puget Group is a geologic group in Washington (state). It preserves fossils dating back to the Paleogene period. Two key formations include the Renton Formation and the Tukwila Formation. See also * List of fossiliferous stratigraphi ...

. It preserves

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

dating back to the

Paleogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago (Mya) to the beginning of ...

period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

. Most of the formation consists of fine- to medium-grained arkosic and feldspathic sandstone, interbedded with lesser amounts of siltstone, sandy shale, coal, and carbonaceous shale. Sandstone typically contains at least 40 percent subangular to subrounded quartz grains, 10 to 25 percent feldspar, and some mica and rock fragments, in a matrix of calcite, clay, and silt. Chert fragments constitute as much as 20 percent of some sandstones. Unoxidized rocks are mostly light gray but are light brown or buff where oxidized. The sandstone is weakly cemented, most commonly with calcite, but in places matrix silt and clay act as the binding agent. The sandstone and siltstone commonly are well sorted and thinly bedded; locally, the sandstone is massive or crossbedded. Some beds in the formation are very soft and plastic when wet, owing to the weathering or alteration of feldspar to a montmorillonitic clay. The Renton formation is a nonmarine

arkosic Arkose () or arkosic sandstone is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose. Quartz is c ...

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

and

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, ...

, with carbonaceous

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especia ...

, and noted for its Muldoon

coal seam Coal mining is the process of extracting coal from the ground. Coal is valued for its energy content and since the 1880s has been widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron fro ...

References

Additional Reading

* Lidar-Revised Geologic Map of the Des Moines 7.5' Quadrangle, King County, Washington , url= https://pubs.usgs.gov/sim/3384/sim3384_pamphlet.pdf= 4 May 2020 Paleogene geology of Washington (state) {{Paleogene-stub