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Geology Rocks Day 5: Serpentine Rocks and Roads

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Day 5 route: You’ll follow the Rogue River until you reach Agness, then climb over Bear Camp Road, which snakes its way between the Wild Rogue Canyon and the Kalmiopsis Wilderness.

The route on Day 5 takes you up the Rogue River to the confluence with the Illinois River near Agness before turning off and climbing the Bear Camp Road to get around the road-less Wild and Scenic canyon of the Rogue River. You will see a wide range of rocks today, along with some wild and beautiful scenery, and end up in the heart of Oregon’s gold country. Gold Beach is so-named because a century ago, gold was being mined from the beach there.

As you leave Gold Beach, you will pass the HWY 101 bridge you came in on, and turn inland. A quarter mile past the bridge, you will pass a spectacular road cut made of serpentine. You will have seen serpentine several times before, but this is a particularly nice example—sea-green to deep blue and shiny.

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Serpentine is a rock made of the mineral serpentinite, and it is a clue that you are looking at rocks that have been very deep in the earth. Serpentine is formed when ultramafic rocks come close to the surface and begin to react with water. Ultramafic rocks (meaning that they contain a lot of magnesium and iron) are part of the earth’s mantle, the rocky body of the earth, which typically begins between 5 and 20 miles beneath the surface. Subduction actually involves jamming big pieces of the earth’s crust into the upper part of its mantle, so it is not surprising that pieces of the mantle get shoved to the surface in the process. These mantle rocks are made of minerals that formed under great pressure and temperature, and when exposed to water as they near the surface, they react to form serpentinite. Serpentinite is soft and weak, and much less dense than the minerals it came from. This means that as it forms, it expands, and actually begins to rise.

Personal experience tells us that rocks are hard and rigid, so it is difficult to visualize how rocks might bend and flow like putty or liquid, but over long periods of time, they do! This means that if there is a body of rock of low density in the earth’s crust, it will rise through the crust, just as a hot air balloon rises through the atmosphere. Soft, low-density serpentine tends to rise through the crust, typically along fault lines. This means that by the time it gets to the surface, it has been repeatedly kneaded, and it is shot through with slip-planes that have been polished to a mirror finish. This is why it appears so shiny.

Of course, it also makes for weak slopes. Shortly after the big serpentine outcrop, you will be riding across large landslides, until you get to Saunder’s Creek (~ mile 3), where you will ride onto river terraces and find some of the most level riding of the day. River terraces form much like the coastal terraces of Day 3. During periods of stable climate and sea level, rivers tend to widen their valleys by eroding from side to side, leaving a layer of sediment across the valley floor. When there is a climate shift or ice age, the river will begin to erode downwards, cutting into the old valley floor and eventually leaving it high and dry, preserving its sediment layer. As with coastal terraces, there are often several levels preserved along a river valley.

Just before you reach the Illinois River, you will cross out of the mélange and serpentinite and landslides, and once again be riding thorough the turbidite sedimentary layers that you saw on Days 1 and 2. You will see distinct layers in the mudstone and sandstone in many of the road cuts as you turn away from the Rogue and start to climb Bear Camp Road.

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Bear Camp Road is a BLM access road that is the shortest route between Grants Pass and the coast in summer, but it’s closed with snow in winter. Tragically, a family was trapped in the snow on this road several years ago when their GPS system directed them over Bear Camp in the winter. After several days, the father left the car to try to get help, descended a steep canyon into the road-less stretch of the river, and died of exposure. The family was subsequently rescued through analysis of weak cellphone pings.

The twisty course up Bear Camp road will include patches of gravel road, the result of constant small landslides that occur in the weak shale and mudstone layers that make up much of the climb. You will pass back into exotic terranes and mélange shortly before lunch. After lunch, you begin a protracted descent, a “serpentine path,” as you wind your way down through the rugged terrain carved in the mélange. You will see a wide range of rocks in the road cuts, with many different colors and textures. Many have been so changed by the heat, pressure, and violence of the subduction process that they are very difficult for a geologist to identify. The last few miles of your descent are along Galice Creek, which was the heart of the local mining district, and which you will see more of on Day 6.

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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2 Comments

  1. Thank you for all the time and effort you’ve put into these blogs. Always fun to learn more, and we appreciate your helping us do that.
    I plan however to be busy with other things during this ride and will not be available to participate in any Tsunami-related activities during the ride. Thank you for understanding.

  2. Wow! I had no idea we get the bonus a geology field trip! Excellent details on the blog. Look forward to renewing our aquaintance on the ride.

    PC BS Geol, OSU, ’84