Late in the Permian, some 230 million years ago (MYA), the western coast of North America curved through what is now western Idaho. Offshore, a volcanic island arc was forming; this volcanism lasted through the Triassic and was followed by a period of sediment accumulation during the Jurassic, when this Wallowa Terrane merged with the western edge of the Continental crust.
Covered by a thick layer of Miocene basalt as the Columbia Plateau formed (15-17 MYA), the Wallowa Terrane would soon lie between the Blue Mountains of Oregon and the Salmon Mountain Uplift of Idaho. Following the rise of the Tetons (about 9 MYA) and in concert with volcanic upheaval from southeast Idaho to northwest Wyoming, the ancestral Snake River began to form, flowing from the east side of the Tetons to the broad, volcanic plain of southern Idaho. There it fed Lake Idaho, which covered the western half of the Snake River Plain during the late Miocene and early Pliocene, draining to the south.
As further volcanism and uplift altered its hydrology, Lake Idaho spilled to the northwest and the Snake River soon occupied that channel, slowly eroding Hells Canyon through the Wallowa Terrane. Reinforced by the glacial meltwaters and copious precipitation of the Pleistocene, the Snake River has sculpted the deepest chasm in North America (8000 feet in some areas), which was widened by the massive Bonneville Flood, 14,500 years ago. Today, the spectacular cliffs of Hells Canyon expose the geology of the Wallowa Terrane, from the Miocene basalt at its rim to the Permian volcanic rock at its base.
