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10. Ancient Waterways: home of Oregon's first salmon

Exotic Terranes

Beginning about 15 million years ago, inland waters became the home to the ancestors of our modern salmon. These ancient rivers and lakes deposited sand and gravel, mud and clay in valleys and basins similar to the landscape of today. Over millions of years, these deposits have been cemented and compacted into soft rock and in some areas have been uplifted from their original position and then eroded away. Many of these rocks formed in the sinking basins of the Basin and Range or in the valleys of the early Willamette, Deschutes, Snake, and Columbia rivers. In many places the rocks are layered with lava flows and volcanic deposits from violent nearby eruptions.

OGDC units: Miocene-Pliocene terrestrial sedimentary rocks, includes the Alkali Canyon, Bully Creek, Dalles, Deschutes, Drewsey, Fern Ridge, Glenns Ferry, Hillsboro, McKay, Simtustus, Trout Creek, and Troutdale Formations

Age Range: 15 million to 2 million years ago
Rock Types: sandstone, siltstone, mudstone, claystone, conglomerate, tuff
Did You Know? The Troutdale Formation in the Portland area contains cobbles that can be traced back to eastern Idaho and western Montana.


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