The Latah Formation is a series of

late Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million ye ...

lacustrine A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from t ...

sedimentary deposits which outcrop in eastern Washington and northwestern Idaho. The lake beds are interbedded with

igneous Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

rock of the

Columbia River Basalt Group The Columbia River Basalt Group (CRBG) is the youngest, smallest and one of the best-preserved continental flood basalt provinces on Earth, covering over mainly eastern Oregon and Washington, western Idaho, and part of northern Nevada. The b ...

. The formation was originally detailed from a site in

Spokane, Washington Spokane ( ) is the most populous city in eastern Washington and the county seat of Spokane County, Washington, United States. It lies along the Spokane River, adjacent to the Selkirk Mountains, and west of the Rocky Mountain foothills, south o ...

by Dr. Kirk Bryan in a 1923 talk, and then formally described in a 1926 journal article by

Joseph Pardee Joseph Thomas Pardee (May 30, 1871, in Salt Lake City, Utah; † March 2, 1960, in Philipsburg, Montana) was a U.S. geologist who worked for the U.S. Geological Survey, and contributed to the understanding of the origin of the Channeled Scablan ...

and Bryan. When first described the formation was thought to have predated the deposition of the Columbia River Basalts, however further investigation showed them to be interbedded, being laid down in successive events. Potassium-argon dating of the formation returned an age range of 21.3 to 12.1 million years old, indicating an Early to Middle Miocene age range. Numerous fossil plants and insects have been recovered from the formation and described. The Latah Formation includes the

Clarkia fossil beds The Clarkia fossil beds (also known locally as the Fossil Bowl) is a Miocene Latah Formation lagerstätte near Clarkia, Idaho. The fossil beds were laid down in a lake roughly 15-million-years ago, when a drainage basin was dammed by the flood b ...

in Idaho.

References

Neogene geology of Washington (state) Geologic formations of Idaho Miocene Series of North America Shale formations of the United States {{US-geologic-formation-stub