Glacial Geology Near McCall, Idaho
|Near McCall, Idaho, three major groups of Idaho rocks border
one another; Granite of the Idaho batholith, flood-basalt flows of the
Columbia River Basalt Group, and metamorphosed island-arc Sedimentary and
volcanic rocks of the Seven Devils Group.
McCall is also at the end of Long Valley, a major tectonic and
structural feature of west central Idaho. The West Mountain escarpment is
the high ridge formed along the west side of the Long Valley fault. West
Mountain and Long Valley are part of a group of linear north-south ranges
and valleys formed by block faulting during the late Tertiary and
Quaternary. The Miocene Columbia River Basalts overlies the gneissic and
granitic rocks of the Idaho batholith's west border and is commonly tilted
15°-30° west. As West Mountain rose and Long Valley subsided, as much as
7,000 feet of alluvium accumulated in the valley.
The braided meltwater streams from these glaciers coursed across the valley depositing sand and gravel. During the older, most extensive glaciations, the braided streams formed the broad, gently sloping area southwest of Timber Ridge that now is the high terrace above the Payette River. The younger, Pinedale age meltwater formed the lower gravelly terrace on which the McCall airport is located.
The glacial deposits are divided into two categories on the basis of
origin. "Till" is deposited directly by a melting glacier as it
forms a moraine. "Outwash" is deposited by the meltwater streams
leading away from the glacier.
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