The sedimentary strata, lying as they did on top of the intruding granite, became a land of lakes and wooded mountains. The climate, which until that time had been sub-tropical, began to approach that of the present temperate zones.

The Miocene was an age of restlessness in the earth’s surface, and era of mountain building, of frequent and violent earthquakes. The Clearwater and Bitter Root mountains, the great Central Idaho granite batholith, were higher than they now are; these high elevations set in motion numerous streams; in many places erosion wore the sediments off, leaving naked granite surfaces staring at the sky.

It was early in this period that the greatest lava flows occurred. They were not usually explosive or volcanic. As the earth continued to cool and contract, the molten lava was squeezed out through long fissures in the mountainsides, spreading in vast sheets and pouring into the valley-moulds between the hills.

Relieved of pressure, the earth lay quiet for a thousand years; vegetation appeared and trees grew to considerable size only to be overwhelmed by another flow. The geologist Kirkham counted twenty-eight successive flows with an aggregate thickness of 4,600 feet. The Columbia River basalt covered some 250,000 square miles of the Great Northwest, including all of the Snake River Plain in Idaho.

That great plain had already been, since the Paleozoic Age, the valley of a stream or lake not less than 4,000 feet deep, and at an unknown altitude. It had long since overflowed, and was cutting a deepening channel to the ocean. Now the lava flows dammed it blow Huntington; again the waters rose to the brim; again a channel cut through reducing the waters behind it to a lake or a succession of lakes in the same big depression reaching back it may be to the Tetons.

Over and over this process was repeated, a period of rest. The appearance of vegetation, tress growing almost to maturity, the rivers cutting their new outlets to the western sea, then a contraction of the earth, great fissures opening up in the sides of the mountains, hideous flaming lava welling out and flowing into every depression and valley. Near Carey, a cataract of molten basalt poured over a granite cliff; rivers of the stuff flowed into rivers of water with such hissings and steaming as Dante never dreamed of. Narrow canyons, worn down by thousand of years of slow patient erosion, filled; ancient Payette Lake, robbed of its outlet, backed slowly up to Glenns Ferry or beyond.

In other parts of the earth, vertebrate land-life had long existed; no traces of the enormous Mexozoic reptiles have ever been found in our State. The eggs of the last dinosaur had been petrified for a million years before vertebrate land-animal life left any vestiges in what is now Idaho.


Sources:
"Idaho: The Place and Its People" by Bryon Defenbach
"History of Idaho" by James H. Hawley