More About Oregon’s Geologic History





The oldest [starting about 300 million years old (myo)] rocks in Oregon lie in opposite corners (here, in blue and purple).  These mountains were laid down when the western-most shore of the continent was at the edge of the blue (now the Wallowa Mountains and Blue Mountains); the purple (now the Klamath/Siskiyou Mountains) was a series of volcanic off-shore islands.  The youngest part of the Klamath/Siskiyous [about 180 myo] is the northern end, at Bandon.

(Then the someday-state was turned almost a quarter-way ‘round, clockwise, with Astoria near the top; and this part of the continent was then near the equator.)

The Low Cascades (here in green) formed from igneous rock from below [about 50 million years ago (mya)].

The older rocks in the opposite corners eroded, and the material traveled down rivers to settle on the near-shore ocean bottom, eventually compressing and cementing to become marine sandstones and mudstones.  That sedimentary rock rumpled up sometime later [starting about 40 mya] to become today’s Coast Range (here in yellow).   The sandstone headlands at Cape Arago are some of that [45 myo] ocean bottom.

Lava flowed up cracks (red spots) all during this time and, near the end of this story [about 25-16 mya], formed the peaks of the High Cascades and flowed in great sheets over areas east of the Cascades and down the Columbia River.

Much more recently [starting about 110,000 years ago], Recent Ice Age sea level changes greatly affected the shape of the coastline (blue line shows the approximate maximum low).

[Years given in square brackets are very rounded off to make them easy to remember.]

What does off-shore Oregon look like?