Geology Rocks – 2022 Joyride

Ian Madin worked as a geologist with the Oregon Department of Geology and Mineral Industries for 32 years and has ridden with Cycle Oregon for over a decade now. His nightly presentations at Classic are popular and, now retired, we’ve asked him to dig in to more of our events to reveal the cool and sometimes hidden geology of the regions through which we ride.

The landscape of the 2022 Cycle Oregon Joyride is gentle, green and welcoming. From the broad flat agricultural lands along the Willamette River to the rolling hills of the Coast Range foothills with their deep green forests, there is little to suggest the violence of the geologic events that shaped it.  By chance as the length of the route increases, so does the depth of geologic time it traverses.  On the short route, you will be riding in a landscape shaped in the last 10,000 years, with glimpses of events as far back as 16 million years.  On the medium route, you will end up over 50 million years in the past, deep in the ocean off Oregon’s coast.  On the long route, you will encounter even older rocks rafted here from distant origins by earth’s ever-moving tectonic plates.

All routes start along a bluff above the Willamette River through quiet residential neighborhoods, then fields and woods.  This surface you are riding on may seem serene, but it was created in one of the most violent geologic cataclysms known.  The road here is laid on layers of Willamette silt, which is a fine sediment that was deposited during the great Ice Age floods (also known as Missoula floods) fifteen to twenty thousand years ago.  At that time the southern edge of the great ice cap that covered the northern hemisphere was near present-day Missoula Montana.  As the ice advanced, it blocked off the Clark fork of the Columbia River, causing an enormous lake to form in the Bitterroot Valley to the south.  Glacial Lake Missoula reached a depth of over 2000 feet, and held as much water as Lake Erie and Ontario combined.  When the level rose high enough, the ice dam floated…and collapsed, releasing an unbelievable flood that swept into eastern Washington, down the Columbia River Gorge, and out the lower Columbia to the sea.  Then the ice advanced and rebuilt the dam, and the whole process happened again, at least 40 times.  The floodwaters backed up into the Willamette Valley as far as Eugene, and each time they left behind a layer of silt, filling the valley floor and smoothing over the existing landscape, including the stretch of road you are riding on.  Here the floodwaters would have been 275 feet deep, and in fact, all of the routes except the highest part of the Long route would have been underwater.

When you turn east onto Buena Vista Road you leave the Ice Age flood surface and drop on to the floodplain and into the Willamette River meander belt.  In the fifteen thousand years since the last Ice Age flood, the Willamette River has wandered across the valley floor carving a series of great arcs and loops into the soft silt in a pattern called meandering. This pattern is beautifully illustrated in the striking image below (produced by Dan Coe at the Oregon Department of Geology and Mineral Industries). What appears to be a curl of smoke in a deep night sky is actually a visualization of the meandering Willamette River channel between Independence and Albany.  Although the land here looks deceptively flat, it is actually riddled with more meander scars and channels, carved by repeated floods of the Willamette.  The largest historic flood occurred in 1861, and covered the entire short route.

As you ride across the meander belt, you will see the Eola Hills to the east across the Willamette.    The hills are made mostly of lava flows of the Columbia River basalt (CRB), which erupted 16 million years ago.  These eruptions were some of the largest to have occurred anywhere on earth and unlike any ever witnessed by humans.  Instead of erupting from a single volcano, floods of 2000 degree lava gushed from huge rifts in the ground in eastern Oregon and Washington.  Time and again, the lava flowed like hot syrup across the landscape, burying everything in its path.  The flows in the Eola hills traveled some 300 miles from their source, as a flowing sea of lava 30-100 feet deep.  

In the Eola Hills those thick dense lava flows sit on weak mudstone.  When the meandering Willamette River undermines the mudstone at the foot of the hills, large landslides are triggered, which then move periodically over decades or centuries.  Most of the western edge of the Eola Hills is a large landslide complex, parts of which are still moving.

While the short route returns across the meander belt and back onto the Ice Age flood surface, the medium and long routes head east into “deeper” water.  Where the meander belt was under 50 feet of water in 1861, and 275 feet of water in the Ice Age floods, the longer routes are headed straight to the bottom of the ocean.  Most of the Oregon Coast Range is composed of sedimentary rocks; mudstone, siltstone and sandstone that were deposited on the floor of the Pacific Ocean between 30 and 50 million years ago. Some layers were deposited in shallow water along the ancient shoreline, and some in the deepest parts of the ocean, but over time several miles of sediment accumulated.  Eventually tectonic shifts began to push the ancient seafloor up, and the accumulated sediment, now compressed into rock, slowly rose from the water and began to be eroded by wind water and gravity.  The Coast Range we see today is the result of that ongoing process of uplift and erosion which carved the soft sedimentary rock into the gently rolling landscape you are riding through today.  Don’t expect to see much evidence of the ancient seas along the way, as the sedimentary rocks are covered with soil and silt from the Ice Age floods.  At best you may see some tan or brown layered rock in the deepest of roadcuts.

The medium route stays in the ancient sedimentary rocks as it continues north on Airlie Road and circles back, crossing back onto the Ice Age flood surface for the last few miles.  The long route ventures further into the geologic past as it climbs into the Coast Range foothills on Maxfield Creek Road.  For the first few miles, the route parallels a major fault line, a break in the earths curst that separates two very different types of rock.  On the right are the marine sedimentary rocks, and on the left, exotic volcanic rocks of the Siletz Terrane.  These rocks originated as a chain of volcanic islands out in the Pacific, much like modern day Japan.  Plate tectonic movements transported them across the ocean to collide with the edge of North America, and the entire block, perhaps 150 miles long and 50 miles wide, welded itself onto the edge of the continent.  The volcanic rocks are more resistant to erosion, so they make for steeper terrain, so perhaps it is a good thing that the long route just skirts around the edge!

Stay tuned for more Geology Rocks, you can bet that Ian will have lots to report on from Classic XXXII and The Painted Hills.

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  1. Really interesting Ian, thank you! I’ve never heard the term meander belt before but now I’ve got a new geologic term I can use with my friends! You did critical important work at dogami not to mention what must have been insanely frustrating politics, thanks for your service!

  2. Clear explanation. I enjoyed reading the article and learning so much. Thanks for writing it.
    The ride (2 days ago) was lovely.