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"The McKenzie watershed is made up primarily of volcanic, sedimentary, and
alluvial geologic regions. The oldest rocks are exposed along the base of the
Coburg Hills and consist of sandstone and siltstone (Water Resources Department,
1991).
The Cascade Range is made up primarily of volcanic rocks. The younger high
Cascades are primarily dominated with lava flows, some less than 500 years old.
Formations of basalts and rhyolites are found in the older western Cascades
(Water Resources Department, 1991).
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Lava Fields off Hwy 126, 10/99
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Alluvial deposits, made up principally of coarse volcanic sand and gravel,
extend far up the McKenzie River Valley. Much of this alluvium has been
deposited along the lower stretch of the McKenzie River to form part of the
Springfield delta (State Water Resources Board, 1991), and other deposits are
found in the Mohawk River and Camp Creek valleys (Meacham, 1990). These alluvial
deposits vary in depth, with older deposits ranging from 100 to 300 feet.
Glacial action is suspected of having placed gravel and rock deposits at
various levels along the upper McKenzie valley. Cemented gravels in river bar
formations are evident at elevations as high as 500 feet above the present
valley floor in the Blue River area."
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