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:: Home > The People > Biological Legacy: A timeline > The Miocene


million years ago

The Miocene and Pliocene Periods of Oregon's Geologic History

The Miocene occurred between 25 and 13 million years ago, and the Pliocene from 13 to 1 million years ago.

During this time, ice ages and geologic activity shaped the forests we have today. The action of North America pushing against the plate of ground underneath the Pacific Ocean produced the Pacific Coast's Coast Ranges over millions of years. (The mountains on the Pacific Coast are the result of more than one geologic event. Some are rocky and volcanic in origin, like the Olympics in Washington, others are metamorphic like the Siskiyous in southwestern Oregon; this diversity is why we discuss the "Coast Ranges" instead of just the "Coast Range.") Cracking of the Earth's crust, beginning about 13 million years ago, allowed volcanic eruptions to form the Cascade Range (molten rock poured from this crack as recently as 1,500 years ago). An ice age from 50,000 to 10,000 years ago covered most of Oregon with glaciers.

During the early Miocene we would find trees that resembled those of modern day, like basswood, elm, and avocado, although these trees are now found in places other than Oregon. Evidence of redwood is found in the fossil record from this time period.

In Pliocene time, volcanic activity gave rise to the Cascades, Klamath, and Blue mountains, as well as to a well-preserved fossil record. The Cascade Range formed a major climatic barrier, becoming the dividing line between moist western forests and dry interior forests and sagebrush grasslands. Ash falling into swamps helped preserve both plants and animals.

Late in the Miocene, nearer to present day, tree species began to look like today's temperate forests of the northern hemisphere, which include willows, spruces, cypresses, oaks, and firs.

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