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 .
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
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.