Geology Rocks Day 2: Landslides, Synclines, and Knockers from Camas Valley to Bandon
Day two of the 2016 Week Ride starts with a long climb up the Tyee escarpment. Near the top, you will start to see outcrops of the Tyee sandstone that holds up this huge ridge.
The Tyee is a particular type of sandstone called a turbidite. Turbidite sandstone forms when submarine landslides cascade off the edge of the continental shelf and flow as a river of suspended sand down the continental slope and out onto the deep ocean floor. The flowing river of sand settles out to form a single thick layer so each sandstone bed you see is the result of a single landslide, and was deposited over the course of a few hours. Some Tyee beds are very thick, which means that the sandstone tends to form cliffs that in turn break up and drop big blocks on the road. Let’s hope you don’t see anything like the 40-foot boulder that recently blocked a road a little north of here!
After you reach the top of the escarpment, you will ride west along a twisting path across the Tyee syncline. A syncline is a geologic structure in which layered sedimentary rocks are folded into a “u” or “ v” shape. The layers on the east side of the syncline tilt to the west, while the layers on the west side tilt to the east. The route snakes across the Tyee because the terrain in this part of the Coast Range is very steep, and most roads end up following along the tops of narrow ridges.
The Tyee is also famous for forming large landslides. You will pass several on your way up the escarpment, but perhaps the most spectacular is at Sitkum, when you descend into the valley of the east fork of the Coquille River. The last mile of your descent is across a massive landslide, one so large that it completely blocked the valley and dammed the river tens of thousands of years ago. The flat valley floor on your right is really the bed of the lake that formed behind the landslide dam, and that eventually filled with sediment. Keep an eye out for irregular terrain and cracks and dips in the pavement, which are good signs of landslides on the move.
You will turn down the river at Sitkum, riding through the narrow gorge the river is cutting through the landslide dam. After riding through the gorge, you will follow the river a few miles to the lunch stop at Frona Park. From there, you will do a series of short hops across low passes between tributaries of the Coquille River until you get to the broad flat valley of the main Coquille River at the city of Coquille. From there, you will follow the river to Bandon.
One of the most dramatic geologic events to affect this area occurred on January 26, 1700, when a magnitude-9 subduction earthquake ripped the Cascadia Subduction Zone from California to British Columbia. About a mile before you arrive at Bandon, you may see the US Fish and Wildlife Service Bandon Marsh Overlook on the right side of Riverside Drive. A trail from the overlook takes you a few hundred feet back to the north along the edge of the marsh, where you can see several stumps in the grass, looking like giant white spiders. These are the remains of old growth Sitka spruce trees that were killed during the earthquake, which caused the land here to sink about five feet, turning the coastal forest into tidal marsh.
Bandon is famous (at least among geologists) for its knockers. The rock underlying Bandon is melange that is part of the Sixes River exotic terrane. Here the melange is composed of a matrix of thoroughly ground up sandstone and mudstone, with big blocks of harder rock that stand out when the soft matrix erodes away. Geologists call these blocks knockers, and the Bandon shoreline is famous for the knockers that line the beach.
After you get to camp, do not miss the opportunity to walk or ride a half-mile south on Beach Loop Road to the Face Rock Scenic Viewpoint. The knockers you can see from here are composed of sandstone, schist, chert, volcanic rock, conglomerate, and blueschist. These blocks may originally have formed hundreds of miles apart, but they have all been brought together here by the relentless motion of subduction over tens of millions of years.
The most famous knocker in Bandon is—or, was—Sae-Tsik-Na (“Grandmother Rock”). This large knocker of blueschist, sacred to the Coquille tribe, was located at the west side of downtown Bandon. It was quarried to build the jetty at the mouth of the Coquille, and none of it remains in place. If you walk to the jetty, you can see the beautiful sparkling blue and green schist, flecked with small red garnets.
Ian Madin is the Chief Scientist for the Oregon Department of Geology and Mineral Industries and in his spare time provides geology blogs for the Cycle Oregon. He will be providing commentary during the evening program and will be a available to answer questions during the ride.