Roadside Geology of Oregon

The Three Sisters in the Cascades, viewed from Eugene, in Oregon's central Wilamette Valley

Roadside Geology (and some Ecology) of Oregon


One thing that struck me, driving back and forth through western Oregon, was how young the landscape was.  The Cascades are still volcanically active (witness Mt. St. Helen’s), and the geologically-recent eruptions throughout that range mean that the soil and vegetation are still relatively simple.  In the last post, I showed you the Belknap Crater, near McKinley Pass, and its surrounding lava flows.

Looking westward from The Dalles along the Columbia River - fog and rain just barely creeps over the Cascades all day long.

That volcano erupted over 4,000 years ago, and very little has colonized the area since then.  Bright green lichen has gained a toe-hold on much of the rock, but pioneer grass and pines are a long time in following, and only a portion of the area has enough vegetative history to have developed a thin layer of soil.

This same story repeats itself throughout the state, as the overlapping volcanic and glacial histories define the landscape and its ecology.   Glaciers of the last Ice Age defined the wide valley of western Oregon between the coastal mountains and the Cascade range, and the eastern second set of mountains,

High Desert, between Bend and Crater Lake

The "pumice flats" are areas of lava and ash where anything has trouble growing

however, and the eastern portion of the state is considered High Desert, barren and sparsely populated, despite the glacial deposits found there.   In between, the Cascades rise,formed by millennia of volcanic eruptions.  Despite having more rainfall than the eastern desert, disturbance by rock-fall, erosion, unchecked winds, and of course molten lava create a  harsh life there for trees and groundcover.

If you look closely, you can see the difference between glaciated peaks and volcanic ones: Mt. Washington (on the left) has been carved by glaciers over the milennia, but Mt. Jefferson (center, in the distance) has erupted since the last glaciation, and stil maintains its cone shape.

In addition to affecting the vegetation, volcanic activity changed the terrain.  Driving through the state, one can still see evidence

Upper Rogue River Gorge

of these eruptions as though they had just ended.  In geological terms, I guess they really have!  Where rivers have carved

Lava flows and collapsed lava tube along the Rogue River

Rogue River flowing over volcanic basalt

through softer layers, the hardened lava almost looks like it is still moving: separate layers of distinct lava flows can be seen in places like the Upper Rogue River’s gorge.  Along the banks of this Wild and Scenic River there is also a very clear example of  the lava tube formation.  In this case, lava flows continuously out from a volcano (sometimes for periods of several months), and eventually the upper crust of the flow hardens after prolonged air contact.  The continuing eruption, however, keeps the flow moving between that hardened crust and the bedrock below.  Once the volume of lava has decreased, and the flow finally stops altogether, the crust

Rogue River pouring out of the lava tube

Top crust of lava tube (the river is flowing underneath it here... see the outflow at the top of the photo

remains, but the tube through which the molten lava has been flowing is empty.  These tubes can (apparently) be miles long.  Most times the crust eventually collapses, leaving deep holes at the surface, or shallow caves when encountered from the side.  In some cases, water, always looking for the easiest way to the sea, breaks through softer rock into these tubes.  This is the case on the Upper Rogue, where the river, diverted by the eruptions, worked its way through both collapsed and entire lava tubes to create the gorge seen today.

Unfortunately, this dam is the reason that only the Upper Rogue is considered "wild and scenic."

I certainly am not a geologist (if you couldn’t tell from the preceding paragraphs), and the volcanic landscape still seems eerily barren to me, but I learned to identify what I was viewing with a basic understanding, and witness these patterns repeated as I went along.  Crater Lake (initially formed by an eruption 7,000 or so years ago) may be one of the more famous locations

Lava spouts at Crater Lake National Park

for viewing the aftermath of volcanic activity, and I spent a day hiking and driving around its rim.  Check out the next post for the stunning detail of that trip!

Secondary eruption wthin Crater Lake

Crater Lake sneak peak!


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