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Geology Rocks! 2023 Classic with Ian Madin

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 presentations at Classic over the years reveal the cool and sometimes hidden geology of the regions through which we ride.

Here is a daily description of the 2023 Classic route, detailing how the landscape and beauty came to be.

Day 1.  Albany to Carlton

Riding north across the floor of the broad Willamette Valley, you won’t see many rocks but you will be riding through a landscape shaped by one of the most catastrophic geologic events known to man. Around 17,000 years ago, the entire Willamette Valley was repeatedly inundated by enormous Ice Age floods that surged down the Columbia River.  The global ice sheet that then covered the Northern hemisphere periodically advanced far enough south to block the Clark Fork of the Columbia near Missoula, Montana, causing an enormous lake to form in the Bitterroot Valley. The lake filled to depths of several thousand feet, eventually rising high enough to float the ice dam, and trillions of gallons of water mixed with shattered ice swept down the river, gouging huge channels out of bedrock across Eastern Washington before charging down the Columbia River Gorge to Portland and then on to the sea.  The flow was so great that the waters backed up into the Willamette Valley creating a temporary lake that reached as far as Eugene. Along most of today’s route, the floodwaters would have been 200 feet deep. At least forty of these floods filled the Valley over a span of several thousand years, each leaving behind a layer of silt and sand, which now make the Valley a perfect place for agriculture of all kinds. Icebergs brought in by the flood left behind hundreds of shallow circular pits where they came to rest, scattering exotic rocks transported from Canada across the landscape. 

Day 2. Carlton to Cloverdale

Crossing the Coast Range will take you through millions of years of geology, most of which is obscured by the deeply weathered soils and dense forest that blanket the land. You may catch glimpses of dark grey or brown exposures of blocky basalt or layered sandstone.  The underlying geology consists of a thick layer of marine sandstone and siltstone that were laid down off the shore of an ancient Oregon between 30 and 50 million years ago.  These layered sedimentary rocks were then invaded by basalt magma rising from great depths, and injecting itself between sediment layers, eventually freezing to form thick bodies of basalt called sills. 

Day 3.  Cloverdale to Cape Meares

The ride along the Three Capes Route to Cape Meares passes a lot of incredible scenery carved from the complex geology.  At Cape Kiwanda you can see a headland carved out of sandstone layers that have been sharply tilted towards the sea.  Cape Meares and Cape Lookout are both large bodies of basalt, which creates headlands on the Oregon Coast because it is far more resistant to the attack of the waves than sandstone. Coming across the Coast Range on Day 2, we passed through basalt sills that formed by being injected into older sandstone. The basalt at these two capes resulted from lava that flowed through ancient valleys in the landscape 16 million years ago. The basalt filled valleys carved in sandstone, and eventually became headlands as the advancing waves removed the softer sandstone.  The really amazing thing about these lava flows is that they were part of the Columbia River Basalt, an enormous outpouring of lava that flooded much of eastern Oregon and Washington. These flows were large enough to travel over 350 miles from vents in far northeastern Oregon to reach the Pacific Ocean, filling the ancient river valleys they travelled with dense black rock.

In many of the hills along the route, the heavy basalt sits on top of weaker sandstone, an instability which leads to giant landslides. You may notice some irregular ground, or even deformed pavement as you cross some of these areas.

Large quantities of sand flow up and down the Oregon coast, driven by waves in the ocean or wind onshore. The bold basalt headlands block this flow, which can cause sand dunes to pile up as you will see climbing from Sand Lake over the crest of Cape Lookout. This dune is much larger than the small area of bare sand you see, the rest has been stabilized by vegetation and is hidden in the forest.

Day 4. Cloverdale to Newport

Today’s ride winds through the Coast Range foothills, again passing through alternating areas of basalt and sandstone. Instead of the basalt sills you passed on Day 2, these are pillow basalts that are part of the Siletzia exotic terrane.  Siletzia is a thick block of basalt that extends along the coast of the Pacific Northwest from Coos Bay to Vancouver Island. It originated as a chain of volcanic islands, much like present day Hawaii, that collided with North America about 55 million years ago and welded onto the edge of the continent.  Pillow basalt forms when lava flows enter deep water. The front of the flow freezes against the water, but the pressure of the liquid rock behind the frozen front eventually forces open a crack through which lava is squeezed, like toothpaste from a tube, freezing solid instantly in the water. This is repeated again and again, building up a thick pile of overlapping sausage-shaped basalt pillows. In some roadcuts, you might see the characteristic rounded shapes of one of these piles of pillow lava.

For much of the ride, you will be passing through the Tyee Formation, which is a thick layer of sandstone that lies on top of Siletzia throughout the Coast range. The Tyee consist of turbidite sandstone, which forms as submarine landslides feed a temporary river of water and sand that flows down underwater canyons to cover the sea floor with a thick layer of sediment. Each sand layer is deposited in a matter of hours, with intervals of centuries or more between events during which a thin layer of mud settles onto the sand. This results in a pattern of thick sand beds separated by thin layers of mud, which you may see in some of the roadcuts along the way. 

Day 5 Toledo to Agate Beach

For the entire ride down the Yaquina River to Newport, you will be within the tsunami evacuation zone for a Cascadia subduction earthquake.  Off the coast of the Pacific Northwest, one of the large mobile plates that make up the earth’s crust is being slowly forced beneath the edge of the continental plate at the rate of an inch a year.  This movement is not constant, the two plates lock up for centuries while stress builds up, then slip suddenly moving 30 or 40 feet in an instant, accompanied by a Magnitude 9 earthquake which violently shakes everything from Northern California to British Columbia. The next earthquake is expected to shake for 4-5 minutes, and be followed 15 minutes later by a huge tsunami wave, driven towards the shore by the violence of the earthquake.  These great earthquakes occur every 540 years on average, with the last one on January 26th, 1700.  Although very unlikely to happen during your stay, you should immediately head for high ground if you feel strong ground shaking. You can download apps to show you how to evacuate from your particular location and its always a good idea when visiting the coast to check out your evacuation route (  for apple for android).

When you reach the turnaround at Agate Beach you will be able to see the lighthouse located on Yaquina Head to the northwest. Yaquina Head is another body of Columbia River Basalt like Cape Lookout and Cape Meares. And yes, there are agates on Agate Beach. Agates are a form of microcrystalline quartz, and are typically translucent and colored in earth tones, and take a high polish in lapidary jewelry. They form when hot groundwater circulates through buried volcanic rock like basalt. The water dissolves quartz from the rock where it is hottest and then deposits it in cracks and open spaces in the rock as it circulates to cooler regions. Basalt flows typically have open spaces called vesicles, which form as gas bubbles in the lava.  The quartz that fills the vesicles is much more resistant to weathering and erosion than the basalt that surrounds it, so eventually the basalt completely melts away, freeing the agate.

Day 6. Toledo-Alsea

Riding south along the Coast from Newport, the route passes across a broad coastal terrace. Coastal terraces form as the result of dramatic changes in sea level that accompany ice ages.  During the peak of an ice age, so much sea water is locked up in ice on the land that sea level drops as much as 400 feet around the world.  At the end of the last ice age 12,000 years ago, the Oregon Coast at Waldport was actually 25 miles further to the west than today’s shore.  As the global ice sheets melt away, sea level rises quickly, and the advancing waves erode a smooth rock platform where they cut into the rocky shoreline. During the next ice age, the sea retreats again, leaving the platform high and dry, but at the end of the next ice age the sea comes roaring back. If the land has been stable, the waves simply reoccupy the older platform, but if the land has been rising due to tectonic forces, the previous platform is now above the waves and a new platform is cut below it. This results in a series of broad terraces rising like stair steps along the coast, and in Oregon there are as many as 5 of these terraces stacked up above the shore.

When you turn inland at Waldport, you will once again pass through Tyee sandstone and Siletzia pillow basalt.  There is a good roadcut outcrop of basalt pillows where Highway 34 crosses Narrow Creek, 4 miles before you reach Alsea.

Day 7 Alsea-Albany

Your last day takes you through familiar bedrock of the Tyee Formation and Siletzia as you climb up the South Fork of the Alsea River, even passing by a few sills of basalt near Alsea Falls. As you drop into the Willamette Valley at Monroe, you are once again riding beneath the high water mark of the Ice Age floods. You will cross the meander belt of the Willamette River on the way to Harrisburg. The meander belt is a broad shallow valley that the river has cut into the Ice Age flood deposits as it wanders along the Valley floor.  The river runs in great loops and curves, which often become cut off to form oxbow lakes. Levees and embankments hold the river in its current channel for now, but eventually we will see a big enough flood to allow it to resume cutting its own sinuous path.  Approaching Albany, the route climbs imperceptibly onto the Santiam Fan, a huge fan of gravel that has been pushed into the valley by the Santiam River, which drains the rugged Cascade Range to the east.  The image below, which stretches from just south of Albany to Independence, uses laser scanned topographic imaging to reveal the many abandoned channels of the meandering Willamette River and the channeled fan of the Santiam River which enters from the right.

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