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Geology Rocks: The Terrain of the 2016 Week Ride

The 2016 Week Ride through the southern Oregon Coast range, along the beautiful coast, will take you through incredibly varied geology—though you may have a hard time seeing it beneath the thick forest that covers the area. This Geology Rocks series will describe the geological highlights of each day, hopefully with some features you can look for along the way. Over the course of the week, you will ride through three very different geologic landscapes, spanning hundreds of millions of years.

You’ll start and end in rocks of the exotic terranes, which are great slivers and chunks of rock that have been plastered onto the edge of North America by ancient tectonic plate movements. These terranes may stretch a hundred miles and be 30 miles wide, and they are composed of a wild assortment of rocks that are 100-200 million years old. As you head west, you will cross an area where the rocks consist of layers of sandstone and mudstone deposited in the Pacific Ocean 50 million years ago. When you reach the coast, you will be riding on a coastal terrace carved by the waves in the last 200,000 years. Lurking offshore is the Cascadia Subduction Zone, a 600-mile-long fault that last moved on January 26, 1700 in a magnitude-9 earthquake. On the layover-day ride to Brookings, you will once again see exotic terranes. You will stay with them as you turn inland to return along the Rogue River with its rich mining history.

Day 1: Ridges and Valleys From Myrtle Creek to Camas Valley

You start the ride following the south fork of the Umpqua River, through a pleasant valley through the Elk River exotic terrane. Exotic terranes are bodies of rock that have been transported hundreds of miles from the areas where they formed by tectonic plate movements.

For hundreds of millions of years, the Pacific Ocean has been steadily sliding beneath the edge of North America in a process called subduction. The ocean floor acts like a giant conveyor belt, moving chains of volcanic islands and other bodies of rock steadily towards the edge of the continent, where they are scraped off as the conveyor belt slides into the depths of the earth beneath the continent. These scraps of the earth’s crust are made of a diverse mixture of rock types, basalt lava, sandstone, mudstone, chert, limestone, and a variety of odd metamorphic rocks. Because these rocks were squeegeed off the ocean floor by subduction, they have typically been folded, contorted, and broken—and in extreme cases, have become a special type of rock called melange. Melange is a French word for mixing or blending, and these rocks have definitely been through the mixer.

Most of what you ride through on day one were originally horizontal beds of sandstone and mudstone, but are now a chaotic mess of ugly grey and black rock, with most of their original form obliterated by the tectonic churn. As you leave Myrtle Creek and head along the river, keep your eyes open for shiny blue green rock in the road cuts. This is serpentine, an unusual type of rock that is often found in areas of melange.

After you leave the water stop in the community of Green, you will head out Happy Valley Road and see a steep, straight ridge in front and to your right. This is the beginning of the much younger sedimentary rocks you will cross on your way to the coast, and your first encounter with the valley and ridge terrain that comes with them.

The sedimentary rocks were all deposited off the coast of North America 50 million years ago in alternating layers of sandstone and mudstone. These layers were originally horizontal, but tectonic forces have tilted them slightly to the west, so as they erode, the hard sandstone layers form long sinuous ridges, while the soft mudstone layers form valleys. You will pass through a narrow notch in the ridge and on into the next valley, where you will have lunch at Lookingglass.

The ridge and valley terrain is common in the Appalachian mountains of Pennsylvania, where the notch you passed through would be called a water gap. This is the first in a series of sandstone ridges that you will pass by on your way to camp at Camas Valley. The biggest by far is the ridge formed by the Tyee sandstone, which stretches for over 30 miles north to south and rises 2,000 feet above the valley. The afternoon-option route climbs the escarpment and then back down. On day two, everyone gets to climb it!

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.

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