The Latah Formation is a series of late Miocene

Miocene

The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

 lacustrine

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

 sedimentary deposits which outcrop in eastern Washington and northwestern Idaho

Idaho

Idaho is a state in the Rocky Mountain area of the United States. The state's largest city and capital is Boise. Residents are called "Idahoans". Idaho was admitted to the Union on July 3, 1890, as the 43rd state....

. Its lake beds are interbedded with igneous rock of the Columbia River Basalt Group

Columbia River Basalt Group

The Columbia River Basalt Group is a large igneous province that lies across parts of the Western United States. It is found in the U.S. states of Washington, Oregon, Idaho, Nevada, and California...

. The formation was originally named from a site in Spokane, Washington

Spokane, Washington

Spokane is a city located in the Northwestern United States in the state of Washington. It is the largest city of Spokane County of which it is also the county seat, and the metropolitan center of the Inland Northwest region...

 by Dr. Kirk Bryan in 1923. Numerous fossil plants and insects have been recovered from the formation and described. Whane first described the formation was thought to have predated the deposition of the Columbia Rover basalts, however further investigation showed them to be coeval.