Geologic Map of Oregon Coast Range.
At its most basic, the Oregon Coast Range consists of a basement of Eocene Basalt, which makes up the so-called "Siletz Terrane", overlain by Eocene through Miocene clastic sedimentary rocks. To the south, accreted terranes of the Klamath Mountains make up the basement. The Siletz Terrane was accreted during the mid- to late-Eocene, concurrently with parts of the Umpqua Group.
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Geologic Map of Oregon
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