Dune science feels kindah religious
Oh sorry, that’s not a dune. That’s a bluff -- no bluffing.
“I have a pet peeve,” said Mark Borrelli, Coastal Geologist for the Center for Coastal Studies, and that’s interesting because Borrelli is not someone you’d imagine having pet peeves; he’s too even-keeled, well-adjusted, and science-driven for that.
“Everyone calls these things ‘dunes,’” he continued as we tromped down the steep sandy face at Wellfleet’s Newcomb Hollow to the beach, surf roaring and foaming before us. “They’re not dunes. Dunes are created by wind. They’re bluffs, and bluffs were created by glaciers.”
So began a guided journey along the jagged line parallel to the Atlantic Ocean where flat sand suddenly rises up, a walk many of us have taken many times.
But in the company of someone spending his life understanding geologic moments, who sees and infers so much more than a casual sun worshipper or beachcomber, the trek becomes something close to religious – invoking creation over eons, musing on mysterious forces that leave telltales for us puny humans to try to decipher.
“Nature is always working to straighten a coast,” said Mark, taking in the long view north and south then picking up a striated chunk that crumbled in his hands -- but hey, if buried under pressure and given 10,000 years or so that chunk could become sandstone like we see in the Southwest.
He figures that as wind and waves do their smoothing, the dune (sorry, bluff) is pushing back at roughly three feet a year.
“That’s just an average,” he added, knowing there are moments when much bigger gouges and collapses take place. “And I’d say the average is increasing because, well, nothing is slowing it down.” Climate change impacts, rising sea level and higher tides, of course contribute.
But high cliffs are no pushovers. They are composed of what Borrelli calls “the three biggies,” in this case actually “the three smallies” – sand, silt, and clay.
Sand is the most easily moved of the three, with the coarsest grain. Where sand dominates, the bluff assumes what geologists call “the angle of repose,” a smoothly sloping, graceful face of brown complexion. As sand blows away the face keeps asserting the angle that expresses the steepest line that can be drawn as the balancing moment when gravity’s tug is equal to sand’s ability to hold in place — repose. Other things like beach grass roots can change the angle, increase resistance and slow movement.
Silt is finer than sand, often seen in rivulets as miniature trickles and streams deposit alluvial sediment like mini-Mississippi Rivers forming tiny deltas.
Then there is clay, “as fine as it gets,” said Borrelli, and he means granular size though for me that also implies fineness as in superfine funky cool. Clay means that thousands of years ago here was a pond, a lake. Often this would be what Mark calls a “low energy environment” where a chunk of glacial ice thunked off, slowly melted, settling to the water table to form a kettle pond fed from below. Clay slowly built up and would “flocculate,” sticking together due to a miniscule electric charge.
“Imagine 20 feet of clay,” said Borrelli, gesturing up. “Think about how long it would take to create that, literally one grain at a time … And when you see striations you can see different periods of time, like tree rings. We’re able to go back 20,000 years.”
Clay with cohesion can stand up to erosion far better than wayward sand. Here is where bluffs become steep, almost vertical. Then dramatic events occur, a huge collapse, many feet broken away at once, because a gradually receding angle of repose was sharpened – until Nature’s straightening force had its way yet again.
Dr. Borrelli is a fine teacher, and I was privileged to get a private tutorial, Coastal Geo 101. But there also was a personal moment, a case study:
When I started this Cape Cod Voice column going on two years ago, I chose as a mascot image a photo of a clay outcrop at Newcomb Hollow that looked much like the profile of an old man. I joked it was my alter ego.
On our recent visit we passed the same spot – and the old man was gone, eroded away.
I’m not taking that as an immediate omen, though it was a reminder:
Time will come when Nature will flatline my lumpy life.
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Dune science feels kindah religious
I think the old man, your alter ego, is still there... it's been photoshopped out. You (and he) have lots of storms and calm days ahead of you before you or he truly flat lines. Just say'in.
Very Informative...and I'm going to add the word "flocculate" to my vocabulary.