The geology of the Upper Wood River Valley is among the most complex in all of Idaho. Massive earth movements, glaciers, volcanoes and subsurface intrusions all played a role in shaping the landscape you see around you today.
Mountain Building

Look around and you'll see three separate mountain ranges converging from the north, east and west. Bald Mountain, home of Sun Valley's famous ski slopes, rises to the west. The Pioneer Mountains lie just beyond the hills on the opposite side of the river to the east. Just north of Ketchum, the Boulder Mountains extend northwest into the Sawtooth National Recreation Area.

These mountain ranges are the product of colliding "tectonic plates" -- 50-mile-deep masses of subsurface rock -- that started bumping into each other some 150 million years ago. As the Pacific Ocean plate slid beneath the western coast of North America in what is now western Idaho, it forced the surface rock upward.

Although older rocks usually lie below successive layers of younger rock, the highest peaks of the Pioneer Range to the east contain some of the oldest rocks in Idaho (2,300 million years). Hyndman Peak is a massive quartzite of pre-Cambrian age. It was bent, folded and thrust heavenward by the mountain-building process of colliding tectonic plates and subsurface intrusions .


Ice Age

The Wood River Valley is a graben -- a large U-shaped valley with a broad bottom created by heavy sheets of ice known as glaciers.

The most recent Ice Age left this area just 10,000 years ago, leaving a few remnant ice fields on Idaho's highest peaks.

Idaho Batholith

The subduction of the Pacific Plate beneath North America continues to this day, causing volcanic eruptions like the one at Mount St. Helens in 1980. Here in Idaho, an eruption occurred underground 50 million years ago creating a huge (18,000 square mile) mass of molten rock known as "magma."

Some of this liquid rock squeezed up to the surface. Most of it stayed well below the Earth's crust, pushing upward against many of Idaho's mountain ranges, making them rise ever higher. Eventually, the magma cooled and hardened. The cooled rock is known today as the Idaho Batholith.

The mountains to the west, including Bald Mountain, are underlain by granitic rocks of the Idaho Batholith. The rocks near the surface are layers of sandstone deposited in an ancient sea more than 600 million years ago.

Outcrops of the Idaho Batholith are easily recognized. The granite has a salt and pepper appearance, made up of biotite mica, hornblende, plagioclase feldspar and quartz.

Digital Atlas of Idaho

Michael Hofferber
Michael Hofferber

phone (208) 886-2963
fax (209) 396-8354
Box 277
Shoshone, ID 83352-0277