The oldest rocks in Washington can be traced back to the supercontinent of Rodinia, which is thought to have formed 1.1 billion years ago. About 750 million years ago (Neoproterozoic Era), Rodinia broke apart and the area just west of Spokane became the shoreline for the continent of Laurentia. The rocks in the eastern portion of the Okanogan Highlands are of Precambrian age (540 million years old and older) and consist mainly of metasedimentary rocks, formed when sedimentary rocks are later subjected to heat and pressure. The most notable rocks formed both before and during this time are the Belt Supergroup, Windermere, and Deer Trail Groups; they extend from British Columbia south to the Columbia River.

This new coast transitioned into an active subduction zone (a subduction zone is a plate boundary where one plate is sinking underneath the other, usually with an ocean plate being crushed beneath continental crust). The Laurentia subduction zone existed from about 350 to 170 million years ago. The Kootenay terrane formed during that time period, which consists of rocks like fossiliferous carbonates, sandstones, and shales that were deposited in the shallow sea near the continent. The Kootenay Arc extends northeastward for 150 miles into British Columbia.

During the Jurassic Period, approximately 175 million years ago, huge amounts of crust composed of micro continents and island arcs within the Intermontane superterrane were accreted to the North American continent. Everything west of the Precambrian rocks in northeastern Washington was created either through complex processes along an active subduction zone or transported from elsewhere and stuck onto the continent. This activity resulted in an arcuate belt of thrust faulting, terrane stacking, metamorphism, and plutonism from British Columbia southward to Arizona.

The Eocene Epoch had a profound effect on the geology of the Okanogan Highlands. Collapse of the orogenic belt that was created in the Jurassic formed large extensional features such as grabens and metamorphic core complexes. A graben is a basin formed between extensional faults. Examples in this region included the Republic, Keller, and Torada Creek grabens. Metamorphic core complexes are pieces of deeper crust that are brought to the surface by low-angle extensional faults, called detachment faults. The Okanogan and Kettle detachments are up to 100 kilometers long. Highly deformed metamorphic rocks, migmatites, and plutons were uplifted along these faults. Eventually, the rocks were exposed at the surface, forming the Okanogan and Kettle gneiss domes.

Diagrammatic cross section across the Okanogan and Kettle gneiss domes, which were exhumed and cooled in early Eocene time in response to extension deep in the Earth's crust. Modified from Kruckenberg and others (2008).

Associated Eocene volcanic activity filled the Republic, Keller, and Toroda Creek grabens with volcaniclastic rocks. In later stages, the basins received stream and lake sediments. Famous fossil lake beds at Republic contain Eocene plants, insects, and fish.

The Okanogan Highlands were covered by great ice sheets during the Pleistocene Epoch. As the ice sheets retreated to the north, lakes formed in the valleys of the Columbia and Pend Oreille Rivers. Along the Canadian boundary, terrace deposits indicate lake levels 2,000 feet above current sea level. Meltwater filled these lakes with sand, silt, and clay.

Glacial Lake Missoula, formed from ice damming of the Clark River in western Montana, repeatedly released massive amounts of water down the Spokane Valley when the ice dam failed, depositing thick sequences of gravel, cobbles, and boulders in their wake.

Regional map of Pleistocene glaciers, glacial lakes, and outburst floods.