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Geology Rocks – Day 3 – Tsunamis, Terraces, and Thanksgiving

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Figure 1: The route takes you south along a broad flat coastal plain till Port Orford, with an option to visit Cape Blanco, the furthest west point in Oregon. South of Port Orford, you will wind along a steep and rocky coast until you reach Euchre Creek, when you will be riding on a much narrower coastal plain until you reach the Rogue River and Gold Beach.

Most of the ride today will be across a broad flat plain that extends from the shore a mile or two inland. This plain is called a coastal terrace, and it is a common feature of tectonically active coastlines. Sea level has changed dramatically during the last several ice ages—dropping to 300 or 400 feet below the current level during the peak of the ice age, and rising to about its current level during the warm periods between glaciations, called interglacials. Coastal terraces form during interglacials as waves steadily erode the shoreline when the sea level is high.

Wave action along the shore eats into the rocky bluffs behind the beach, carving a flat plane across the rocks that lie beneath the beach. As the shoreline moves inland, the rock plane is slowly buried by sand as the water deepens above it. Coastal terraces are preserved because tectonic forces are pushing the land up slowly, so that a terrace formed during an earlier interglacial period of high sea level is lifted above the maximum sea level reached in the next interglacial. The youngest terrace you ride on was formed about 80,000 years ago, during the most recent interglacial. We are now about 12,000 years into the current interglacial, and sea level is rising at about 1/10th of an inch per year in southern Oregon, so the modern shoreline is steadily eroding into the 80,000-year-old terrace.

If you visit the Face Rock overlook, you can actually see the thin layer of sand that sits on top of the planed-off rock. In places, you can see the 80,000-year shoreline where it was eroding into the previous terrace, formed during an interglacial 105,000 years ago. You can also see the shoreline from 105,000 years ago where it was cutting into an even older terrace or the mountainous bedrock of the Coast Range.

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Figure 2: After turning inland at Bradley Lake, you will be riding on one of a series of coastal terraces, shown in shades of purple. Each terrace is older, higher, and further inland than the previous. The terraces are covered with sand dunes in places (yellow). You will pass numerous cranberry bogs along this stretch (shown in red), and the hills inland of the coastal terraces are melange bedrock of the exotic terranes. The many resistant knockers in the melange are visible as bumps of varied shapes and sizes.

After a few miles riding along the seaward edge of the terrace, you turn inland at Bradley Lake.  This small lake is quite famous in the world of geology, because it is the best-known natural tsunamometer (tsunami recorder). The lake formed because a migrating sand dune blocked the mouth of a small creek. The outlet of the lake is high enough that waves never get into the lake, even during the biggest storms. However, the tsunami waves that followed prehistoric megathrust earthquakes on the Cascade Subduction Zone were large enough to enter the lake and flood it with seawater and beach sand.

This disturbance left a characteristic layer of sediment on the lake bottom, and core samples of these layers allow geologists to determine how many tsunamis have occurred over the last few thousand years. The lake records 14 magnitude-8 and magnitude-9 earthquakes in the last 5,000 years. Oregon coastal communities are preparing for the next great earthquake and tsunami, and you may notice blue signs announcing that you are entering or leaving the tsunami hazard zone as your ride along.

Shortly after you turn south onto Highway 101, you will see white and tan colored sand with orange iron oxide stains in the road cuts. This is the sediment deposited behind the advancing shoreline when this terrace was formed 80,000 years ago. You also will see large rectangular ponds along Highway 101, though they may be dry when you pass them. These are cranberry bogs, which take advantage of the climate, abundant water, and soil conditions on the coastal terrace. Oregon produces about 40 million pounds of cranberries each year and is the fourth-largest-producing state. A substantial part of the terrace surface has been modified to make the bogs.

The coastal terraces get narrow as you head south, and you will see more of the mountainous terrain to the east. The rock here is part of the exotic terranes, and is once again melange, strewn with resistant knockers of all sizes. If you take the optional ride to Cape Blanco, you will see some beautiful sea stacks formed by resistant knockers. Cape Blanco is named for the cliffs of white mudstone that make up part of the Cape. These rocks were deposited in a shallow bay as recently as five million years ago.

South of Port Orford, you will pass high road cuts of reddish brown sand and clay, which are remnants of very old coastal terraces that have been lifted 600 feet above the modern sea level.  You will continue to follow the coast for a while, then the highway turns inland at Humbug Mountain, which is a giant knocker of resistant conglomerate, or cemented gravel. After you rejoin the coast further south, you will pass across large active landslides for more than a mile, and you may notice the dips and bumps in the road caused by past movement.

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Figure 3: The coast becomes steep and rocky south of Port Orford, and you will have to turn inland to get around Humbug Mountain, which is a very large knocker in the melange (shown in green). Just south of the mountain you will cross a series of big landslides (shown in lavender).

When you pass Euchre Creek, you will once again be riding on the coastal terrace, although it is much narrower here. Have a careful look to your left as you turn in at the Rogue River to cross the bridge—there is beautiful shiny blue-green serpentine in the road cut. You may also notice blue signs announcing that you are the entering the tsunami zone.

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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