The Palouse River headwaters are born in the clear stony brooks of Idaho's Hoodoo and Clearwater mountains and fed by tributaries emerging from the Thatuna Range located between the river's north and south forks. These eastern uplands are composed of the western buttes' parent belt quartzites and argillites that rose with the Rocky Mountains when the Cascade Range had not yet emerged above the Pacific waters. In the formative processes of this early Mesozoic Age of explosive Rocky Mountain strato-volcanoes, hot magmatic fluids under great pressure penetrated this younger earth's crust and brought certain metals in gaseous state nearer the surface to form soluble compounds like gold chloride and aluminum-iron silicate. In places where water penetrated to great depth these compounds dissolved, mixed with the magma, and were forced through fissures with other solubles like silicon dioxide, or quartz, to create veins containing precious metals and alamandine garnet crystals.
Where this petrographic drama transpired under ancient weathered surfaces as in the Hoodoos, these deposits were worn by water until soft yellow flakes, larger nuggets, and violet-red gemstones fell out into streams which usually held these heavy particles near their sources. As in other high places along the Pacific Slope, indications of this placer gold in North Idaho resulted in nineteenth century regional rushes as prospectors flocked to the rumored El Dorados. Dodecahedron-faced garnets and rainbow-colored "harlequin" opals have also been sought in the eastern Palouse as the region's only semi-precious stones.