This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
One thing you'll notice when visiting Puget Sound's lovely shorelines: they're definitely bluffing. Just about every beach here demands a strenuous workout before you get to dabble your toes in the water. Are you standing in the surf right now? Great! (Cold, innit? I'm afraid our seas aren't warm. Sorry.) Turn landward. Do you see a steep cliff sort o' thing looming behind you, wearing some greenery and promising you some serious exercise when you exit your seaside siesta?
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
Yeah, that's a bluff.
It's estimated that more than 60% of our shorelines have got 'em. There's all sorts: tall and short; super-green and basically-brown; nearly vertical and pretty laid back. Some of them display their stratigraphy like they were made for a Geology 101 class, while others hide it behind slides, slumps, or twelve billion plants. Some of them do their utmost to convince you they're sea cliffs, while others pretend they're just hills. In short: they're a diverse bunch.
Let's say yours is very up-and-down.
What, you may ask as you eye the very steep landform you'd need proper mountaineering gear to climb, is the difference between a cliff and a bluff? I don't see any!
That's why the words cliff and bluff share custody of the same basic types of landforms, but a bluff has got a few traits you don't necessarily associate with a cliff. When you see a cliff-like thing or marked slope eroded into stuff that hasn't yet turned to stone, and has got lots of plants and soil all over it (probably hiding some of the lovely strata), it's probably a bluff. Around the Puget Sound area, we generally call water-side slopes carved from glacial sediments bluffs, and grant near-vertical rock faces the name cliffs. We've got lots of those, too - they'll probably concern us at a later date, knowing me and rocks.
Most of our bluffs measure vertically somewhere in the neighborhood of 50-500 feet (15-150 meters) high. The bluff face itself can be a lot more than that, depending on the angle of the slope. Almost all of them are carved into glacial material: layers of sand and clay and silt and pebbles piled thick atop bedrock or interglacial sediments. Geologists think our bluffs started becoming their modern selves after the ground recovered from being weighed down by tremendous volumes of heavy ice when the continental glaciers receded. Picture what happens to a memory foam mattress when you get up - it's kind of like that, only very very slow (by human standards). That process - isostatic rebound - finished sometime around 5,000 years ago. Like most coastal cliffs and bluffs, ours are geologically young.
And they're not going to be around for long.
King County, where Seattle is, boasts 113 miles (182 kilometers) of lovely shoreline. 58% of that scenery is unstable. Even rocky coastal cliffs are vulnerable to time and tide (not to mention weather, chemistry, physics, and folks); how much more temporary a bluff carved into unlithified glacial and interglacial sediments is! Haven't got a chance. Enjoy 'em while they last, because our bluffs are getting carved up fast.
But you know, that's actually a good thing.
Okay, maybe not so great if you happen to live on one of those fast retreating bluffs, but this is the way the geology goes. Beaches need crumbling bluffs. They're depending on bluffs to deliver a good portion of their material. Our own geologist Wolf Bauer, noting how much our bluffs nourish our beaches, even coined a term that's so far pretty unique to Puget Sound: feeder bluffs. He found just the right words, because our disintegrating bluffs definitely feed our beaches: when bluffs are prevented from eroding normally, those beaches starve. Some of those bluffs are so important to the system they've been termed "Feeder Bluff Exceptional" - which Discovery Park's South Bluff certainly is.
Those are our bluffs: dangerous and delicate, dramatic and delightful. Next, we'll begin exploring the bluff that exemplifies our lovely landforms: South Bluff. Get ready to get your feet wet!
References: