Geology Is Cool: West Mountain

Cliff Face, Summit of Snowbank Mountain, West Mountains, Idaho

Cliff Face, Summit of Snowbank Mountain, West Mountains, Idaho

This is an eastward-facing cliff face, near the summit of Snowbank Mountain, a part of West Mountain.1 It’s about 100 vertical feet of rock. The whitish rock chunks across the top quarter were limestone, which would have been laid down in a horizontal bed on a sea floor. The dark gray rock was serpentine, a component rock of ocean bottom.

What’s this stuff doing almost 8,000 feet up in the air?

The geology of West Mountain is about as complex as geology gets in Idaho. Geologists are still puzzling it out. But it seems to have gone something like this:

About 105 million years ago, this was the western edge of North America; the ocean that would become the Pacific lapped against this area. Off shore, there were some big, volcanic islands, what geologists call a “back island arc,” that had originated much further west and south, but were carried east by plate tectonics to the old west coast of North America. Think something like the Phillipine Islands. The ocean floor continued to sink under the Idaho coast, generating the kind of volcanic mountains you see in the Cascades today, until about 90-100 million years ago. The islands then did a very slow motion crash into North America. Those islands, called the Seven Devils Complex today, were a mix of volcanic rock, sand and corals, and some sea bottom scraped up as the complex moved across the Pacific.

When the Seven Devils Complex collided with the old Pacific coast, the rocks that were between them got smashed up pretty good. The heat and pressure were enough to cause metamorphic rock to be created out of the volcanic and sedimentary rocks that had been on the islands and the old coast line. Sea bottom stuff got squeezed up like toothpaste. Horizontal layers of rock got folded and refolded. The collision finally ended about 8.5 million years ago, but the geology had just begun.

Parts of the old subducted sea floor and surrounding rock melted and erupted as basalt on the surface. Other parts didn’t quite make it to the surface, but were left to gradually cool. When erosion exposed them, we had the Idaho batholith. But not before immense amounts of basalt got laid down, in places as much as 2,000 feet of the stuff. Much of the old Seven Devils Complex and the former Idaho coast line were simply buried under the floods of basalt. Some parts stood out above the lava floods, including the Seven Devils Mountains on the east side of Hell’s Canyon, and the Wallowa Mountains on the west side. They are buried up to their hips in basalt. The Snake River, possibly re-routed by all that basalt, carved Hell’s Canyon down through the rock.

Geology wasn’t done yet. Next, the Basin and Range process started, as parts of the new North American coast attempted to head north. All across the west, the stretching caused parts of the the intermountain west to move like stupendous trap doors, creating steep scarps on one side and long, sloping stretches on the other. West Mountain is the northwesternmost expression of the Basin and Range. The east side was lifted up like a partially opened trap door. Sediments eroded off the steep side, creating Long Valley to the east. When West Mountain lifted up, it carried all that old rock with it, which is why you find ocean bottom 8,000 feet up a mountain.

Finally, the series of relatively recent ice ages created mountain glaciers that carved up the summits. In the case of the area from Cascade north, there was enough ice to generate a valley glacier that carved out the upper valley and dug a gouge that is now filled by Payette Lake.

The metamorphosed rock cooled quickly enough that the when the sandstone got heated and compressed into quartzite, the mineral created largish crystals. Here’s WC’s first macro photo:

Squeezed and baked sandstone after 10 million years in the overn

Squeezed and baked sandstone after 10 million years in the oven

All that action, all that geology, in just that small area over just the last 2.2% of the Earth’s existence. It makes you wonder what will happen next.


  1. Technically, it’s West Mountains, but locally its called  “West Mountain.”