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16. Glacial Deposits: runaway global cooling

Exotic Terranes

Ice ages can last tens of thousands of years. Ice caps grow at the poles and push their way toward the equator in great sheets that can be over 2 miles thick. In mountain ranges away from these ice sheets, smaller glaciers cover mountain tops and fill valleys with rivers of ice that carve out rounded valleys and leave behind great piles of rock rubble called glacial moraines. Rock flour ground by the ice is carried by wind and deposited as thick blankets of silt called loess. Although the great ice sheets have never made it as far south as Oregon, mountain glacier deposits can be found throughout the Cascade and Blue Mountains and on many of the isolated mountains and ranges in eastern Oregon. Remnants of the great mountain glaciers cling to a few of the highest Cascade peaks, and loess blankets large areas around Pendleton and Portland. About 20,000 years ago, so much water was frozen in ice sheets and mountain glaciers that sea level was nearly 400 feet lower than it is today. The Oregon coast was located some 10 to 40 miles west of the modern shore.

OGDC units: glacial deposits, glacial outwash deposits, loess

Age Range: 2 million to 10,000 years ago
Rock Types: gravel, silt
Did You Know? Glaciers moved 18 million cubic yards of gravel sand and clay to build the moraines that surround Wallowa Lake. That’s 1.8 million dump truck loads.
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Wallowa Lake


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