Glacial Geology Near McCall, Idaho

(under construction)

 

Introduction:

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 broad, high elevation region north of McCall was mostly buried by an ice cap during Pleistocene glaciations. At the same time, cirque and small valley glaciers formed on West Mountain. During at least three periods of glaciations, major valley glaciers flowed from Ice cap in to the north end of Long Valley and formed large arcuate moraines. Most recently, during the Pinedale Glaciation, the North Fork valley glacier carved the basin and deposited the moraines which form Payette Lake, and the Lake Fork valley glacier formed the moraine of Little Payette Lake. During earlier glaciations the valley glaciers were thicker and longer, forming the prominent medial moraine, Timber Ridge.

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.
The older moraines and outwash plains are not only distinctive because of their position farther out in the valley, but surfaces of these older landforms have been eroded and weathered than the younger moraines and outwash. The greater weathering is revealed by examining the soils. 

 

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