Rides

No Comments

Geology Rocks – 2022 GRAVEL

Ian Madin worked as a geologist with the Oregon Department of Geology and Mineral Industries for nearly 30 years and rode with Cycle Oregon for many of those years as well. 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 all ride. Ian will not be riding with us for GRAVEL this year but has given us this extensive review of each day’s route. Thank you, Ian!

Stone from the Sea

The geology of the 2022 GRAVEL routes is well-hidden beneath the rich blanket of forest that covers the Oregon Coast Range.  Thousands of years of heavy rain have weathered the rocks to soft masses of silt and clay, and rock outcrops are rare.  Still, geologists have scrutinized every logging roadcut and bushwhacked down every creek bed to put together a clear picture of the geologic history of the area. 

As interesting as the details of that story are to geologists, the big picture is very simple; from about 50 million years ago to 16 million years ago, western Oregon was under the Pacific Ocean, and a thick series of sedimentary rocks were slowly laid down.  Sometimes the water was shallow and sandstone was deposited, sometimes it was deep and mudstone was deposited. Sometimes the sediment came from the ancient Cascade volcanoes to the east and was rich in volcanic ash, other times it came from rivers draining granite mountains in Idaho and Montana and it was rich in shiny flakes of mica. 

The rocks hold fossils of marine organisms large and small that record the steady progress of evolution, and even contain beds of coal that were mined by past generations.  However, most of what you will see on this ride is some grey, tan or brown layered rock in the deeper roadcuts, and the dense forest will obscure the ways in which the geology shapes the landscape. 

Day 1 – Saturday May 21

Your ride starts in Toledo, located on the Yaquina River just a few miles inland from the Pacific Ocean.  Despite the distance from the ocean, the river here is still influenced by the tides.  Toledo is also within the inundation zone mapped by the Oregon Department of Geology and Mineral Industries for a tsunami triggered by a future Magnitude 9 earthquake on the nearby Cascadia Subduction Zone.

As you leave the Yaquina River behind and begin your climb into the hills, you will be entering a landscape dominated by the Tyee sandstone, and virtually all of both day’s ride will be in this same geologic unit.  The Tyee sandstone is one of the older layers in the Coast Range, and covers vast areas.  While most sedimentary rocks deposited in the sea accumulate slowly and steadily, the Tyee has a much more exciting history. It is a turbidite sandstone, meaning that each layer was deposited within a few hours by a submarine turbidity current. 

Turbidites occur when there are submarine canyons that connect the deep ocean floor with the shallow waters of the continental shelf (see figure below).  Sediment from rivers accumulates on the shelf around the heads of the canyons until a storm or earthquake triggers a landslide down the canyon. The landslide quickly becomes a flowing mass of sediment and water called a turbidity current, which slows down and spreads out when it reaches the bottom of the canyon and the flat floor of the deep ocean. As the current spreads and slows, the sediment it carries is deposited, starting with the coarsest sand, becoming progressively finer as the current fades. This produces a characteristic sequence of layers, with a thick sandstone base grading upwards to fine siltstone and mudstone.  A turbidite is deposited in a matter of hours, and then centuries or millennia may pass before the next layer is deposited, during which very fine mud slowly settles out of the water column onto the top of the turbidite.

Turbidite layers form when underwater landslides turn into flows of water and sediment. The flows originate on the shallow continental shelf, and are deposited on the deep ocean floor.

This means that the Tyee sandstone is composed of alternating layers of strong sandstone, separated by slick weak mudstone, which is a recipe for landslides.  Large landslides are common through the Tyee sandstone, and along both days of your ride.  In most cases you won’t see them, because their characteristic irregular terrain will be hidden in the forest.  However, the common occurrence of unstable slopes is part of the reason that most of your ride will either be along a ridge crest or a valley floor, where roads are least likely to be impacted by landslides.

When you reach the community of Ona, you will pass out of the Tyee sandstone into the Nestucca, Alsea and Yaquina Formations, which are younger sedimentary rocks deposited in different parts of the ocean floor.  You aren’t likely to see any exposures of these rocks, but if you do they will likely be brown or gray layered rock visible in the deeper roadcuts.

Roadcut exposure of Tyee turbidite sandstone along highway 20.  Each white layer is the sandstone base of a single turbidite layer, while the dark layers are the upper mudstone part. Note the car for scale, some of the turbidite layers are 10 to 15 feet thick, each deposited in an afternoon.

For the last few miles before you reach the beach, you will be riding along the valley floor, and the hills on either side will give way to broad flat plains.  These are marine terraces which record the glacial history of the planet over the last half million years.  The great ice age that ended 11,000 years ago was only the most recent in a long series of glacial and interglacial periods that go back 2 million years.  During the peak of each ice age, so much water becomes locked up in ice on the land that the sea level around the world is lowered dramatically.  During the most recent ice age, sea level was about 400 feet lower than today, and the Oregon coast at Ona beach was 40 miles west of its current position.

When the ice melts and seas rise, the shoreline moves rapidly inland, and where the rising waves encounter hills, they cut a flat bench into the rock.  The bench grows steadily toward the land, while the older parts get covered with sand as the water above them gets deeper.  Eventually sea level reaches a stable high stand and the shoreline stops moving.  During the next ice age, the cycle repeats, and if the height of the land has not changed, the waves re-occupy the previous bench till they arrive at the old shore.  However, if the land has been rising the bench cut but the first high stand will be above the level of the second high stand, and a marine terrace will have been preserved.  If this happens repeatedly, you end up with a series of terraces lined up along the shore like a flight of stairs, with each step older than the one below.  In Oregon there are as many as four terraces. The youngest was formed during a high stand 80,000 years ago, followed by one at 100,000, 120,000 and 250,000 years ago. The terrace at Ona Beach is the youngest, known to geologists as the Whisky Run terrace.

If you have taken the long route you will pass by Alsea Bay at the mouth of the Alsea river on your way to Ona Beach.  Take a look across the bay and imagine what this would have looked like 11,000 years ago at the peak of the last ice age.  Remember, the shore would have been 40 miles west, so here the Alsea River would have been flowing in a canyon several hundred feet deep where today you see only mudflats.  

Day 2 – Sunday May 22

Today’s route once again traverses nothing but Tyee sandstone, though riding through a pile of ancient turbidite layers is a good segue into talking about modern turbidites in Oregon, and the amazing story they tell.  Lurking off the coast of Oregon and Washington is a giant active fault called the Cascadia Subduction Zone. This 600-mile-long fault lies in the depths of the Pacific Ocean 60 miles offshore, but is poised to produce a Magnitude 9 earthquake at some point in the not-too-distant future.

Geologists initially discovered evidence of the most recent great earthquake by examining “ghost forests” of dead cedar trees in tidal marshes in Oregon and Washington.  The Cascadia earthquakes are so powerful that they cause parts of the coast to drop as much as 6 feet during the earthquake, which put the low-lying cedar forest into the range of tides, killing the trees.  These great earthquakes also produce enormous tsunamis, which may inundate the Oregon coast with water as much as 100 ft deep.  Dating of the tree rings in the ghost forest coupled with the discovery of a tsunami recorded in Japan pegged the date of the most recent earthquake as January 26th, 1700.

Sediment core from the ocean floor off Oregon showing alternating layers of sand and clay, each representing a turbidite triggered by a Cascadia Subduction earthquake.

However, the earthquake records in coastal marshes only go back a few thousand years, because these areas have only been near sea level for a little while. Remember that sea level was 400 feet lower 11,000 years ago, so older records are now deep underwater. This is where turbidites come in. Scientists recognized that earthquakes in Cascadia were likely to cause turbidites that would collect on the deep sea floor off the coast.

Today’s route doesn’t have the visibly extraordinary features that we’ll see later this year in the Painted Hills but that just leaves you more time to enjoy the ride itself. You don’t need to be a geologist to appreciate the absolutely gorgeous ride along Big Elk Creek to wrap up this great GRAVEL weekend. Have a great ride!


Leave a Reply

Your email address will not be published.