Oregon’s coast is a geologic wonderland of steep cliffs, nearshore sea stacks and caves, estuaries where rivers flow from the coastal range into the sea, and a large variety of landscapes and rock types. The entire coast is adjacent to the Cascadia Subduction zone where the oceanic plate is colliding (and descending beneath) the continental plate. Because the plates are converging, it is not surprising to see evidence that the land has been uplifted by compressive forces. In July, Jay and I spent a few days in Brookings, the best example I’ve seen (so far) of uplift along Oregon’s coast. To learn more about the tectonic setting of southwest Oregon, please see this previous post: https://landscapes-revealed.net/how-does-the-rogue-valley-fit-into-the-larger-tectonic-picture/.
Brookings, Oregon
Located just 10 km (6 miles) north of the California border (white east–west line on map), Brookings is a town with a population of around 7,000. Because of the rugged terrain of the Klamath Mountains, the trip from Ashland where I live (upper right corner of map) is indirect and about 233 km (140 miles) long. The route goes northwest on Highway I-5 to Grants Pass (north edge of map), then southwest along U.S. Highway 199 through Cave Junction to Crescent City (lower left corner of map), then north on Highway 101 to Brookings. The coastline at the south end of Brookings is indented to create an embayment that protects the town from direct onslaught of swell waves approaching the coast from the northwest. Two rock jetties (in the distance beyond the large sea stack) further help to protect port facilities that were built where the Chetco River enters the sea. We stayed at the Mermaid Muse B+B that has a fabulous coastal view to the south. Jay is enjoying a glass of wine provided by our gracious hosts.The Samuel H. Boardman State Scenic Corridor is located along the coast a short distance north of Brookings. This gorgeous stretch of coast features craggy bluffs, secluded beaches, and offshore rock formations. The Oregon Coastal Trail provides access for hiking along the 30-km (18-mile) length of the state park.
Marine terraces
Marine terraces—elevated flat surfaces on coastal cliffs—are the best indicator of coastal uplift. For more information about how they form, see my previous post: https://landscapes-revealed.net/the-geology-of-point-reyes-national-seashore/. In brief, movement on faults such as Oregon’s subduction zone can cause the continental plate to be pushed up. Platforms that are cut by waves at sea level get raised to higher elevations with each movement. The old, uplifted platforms are called terraces. Their age can sometimes be determined by analyzing ancient beach deposits that overlie the old platforms.
This diagram illustrates modern and old platforms cut into bedrock (olive green color) by waves. Colluvium is sediments deposited on the platform by erosion of the paleo (old) sea cliff. Earth is currently in an Ice Age with alternating glacial and interglacial periods. Platforms cut during interglacial periods with high sea level are abandoned during glacial periods when sea level is much lower. When sea level rises again—during interglacial periods like today—fault movements have uplifted previously-formed platforms and waves must then cut new platforms at sea level.
At least five levels of marine terraces are visible at Brookings. They can be recognized as flat surfaces that indent the seaward-facing slope.
This view of the south end of Brookings shows four terrace levels (yellow lines). The top of the ridge is the oldest abandoned wave-cut platform; as faults uplifted the coastline, more platforms were formed at sea level and successively uplifted. There were certainly more platform levels, but some of the terraces have been completely eroded away.
What is the rate of uplift along the coast, you may ask. There is a great deal of uncertainty—mainly because it is very difficult to obtain ages for the various terrace levels, but estimates range from 0.1–1 mm/yr. This is an average, based on many earthquakes. For example, let’s assume that a large-magnitude earthquake occurs every 300 years and causes 150 mm (15 cm) of uplift. This would equal 0.5 mm/yr, on average. This seems like a very slow rate, but in 300,000 years, there would be a total of 150,000 mm (150 m / 500 ft) of uplift. The terraces at Brookings may be the result of around 1 million years of uplift.
Here is a closeup of one of the lowest (youngest) marine terraces. My feet are just below the contact between the wave-cut platform and overlying beach sediments (see diagram above). The platform is cut into bedrock (greenish color) that is more than 60 million years old. The sediments are much younger—they were deposited at sea level when the platform was cut, probably about 100,000 years ago. The location is Chetco Point and the view is southward. Photo courtesy of Jay.When driving in Brookings, the uplifted platforms (marine terraces) are easy to see. Left-side photo. Uphill view on Oak Street, at the corner with Pine Street and Stout Park. I am standing on one terrace, the black SUV is on a higher (older) terrace, and cars at the top of the street are on a yet higher terrace. Right-side photo. I turned 180° and took a photo from the same terrace but with a view toward the sea. The car stopped at the light is on a lower (younger) terrace, and an even lower one is visible in the distance.Sea stacks are abundant along the modern coastline (see photos at top of page), and they also formed along ancient coastlines. This photo is from Azalea Park in Brookings. The flat surface is one of the marine terraces and the little hill is an ancient sea stack that formed when this was an active wave-cut platform at sea level.
Other evidence of uplift
This map of Samuel H. Boardman State Scenic Corridor shows coastal highway 101 (in black) and the Oregon Coastal Trail (in red). Location 4 (in red circle) is Indian Sands, so-called because of extensive sand dunes that have been uplifted to high elevations.If you are familiar with the Oregon coast, you probably know about the active sand dunes—called Oregon Dunes—located on the coast south of the town of Florence. This photo at Indian Sands shows extensive sand dunes that have uplifted far above the coastline. The hard surface Jay is standing on is a paleosol—the remnants of an ancient soil that developed on the dunes. This photo at Indian Sands shows a cross section of an ancient sand dune. The angled layers are from sand grains that cascaded down the front of a dune. My hand is at a contact between the dune sand and more wind-blown sand deposited on top of the dune. The reddish color at the top is the ancient soil layer. Plants that grew in the sand disrupted sand grain layering. Photo courtesy of Jay.
Geologic hazards
The Oregon coastline is subject to a variety of hazards. Large winter waves undercut coastal cliffs and threaten structures built on the cliff tops. Subduction zones produce Earth’s largest-magnitude earthquakes—during such an earthquake, structures along the coast would experience intense shaking. Subduction zones also generate the largest tsunamis—following a large-magnitude earthquake, waves many meters (10s of feet) high could attack the coast.
I took this photo at Harris Beach in Brookings. It is surprising to see that large new homes are being built at this location. The one on a terrace remnant is at least at a high elevation, although it’s being built very close to the cliff edge. The houses being built at low elevations are more susceptible to destruction by waves. In California, the Coastal Act of 1976 (mostly) prohibits this type of development, but laws in Oregon are apparently weaker. A recent (2024) book by Rosanna Xia—”California against the sea; visions for our vanishing coastline”—describes how the California law got passed. There are still many problems, however, because some homes were built too close to the sea before the Coastal Act was passed.
The marine terraces at Brookings keep many homes and other structures at relatively safe elevations above the sea. Many parts of the Oregon coast are at lower elevations, however, and structures built there are threatened by imminent earthquakes and continuing sea level rise.
Glad you had such good weather!
As a fog-a-phobe, it was honestly a little cool for me, but we did have sun too! We really liked the B+B on the cliff top.