Reading Rocks - Grand Falls
In Grand Falls, New Brunswick, the St John River drops 23 meters into a beautiful, sheer gorge. And by reading the rocks of that gorge, we can find three stories of ancient seas, glacial upheaval and the surprising power of erosion.
EPISODE NOTES
If you’re wondering why Jean-Baptiste de la croix de Chevrières de Saint-Vallier was so controversial: he never listened to advice and left the seminary in great debt at the time of his death.
While filming early in the morning in Grand Falls, I kept thinking I was catching glimpses of a fox in the underbrush as I made my way up and down the gorge trail. I spent longer than I’d like to admit setting myself up to be ready to film him, only for him to be… a cat.
The episode also does not mention a descent of 401 steps you can (and I did) take to get down nearly to the water level. I wouldn’t say the gorge looked any more majestic from down there, but you can take the 401 stairs for the sake of saying that you took 401 stairs.
Here’s a panorama of the gorge!
TRANSCRIPT
May 16th, 1686. Jean-Baptiste de la croix de Chevrières de Saint-Vallier, the controversial second bishop of Quebec, writes the following:
‘we arrived at a place called Grant Sault St-Jean-Baptiste. Here the river falls madly from a height of 60 feet, forming a huge waterfall that thick fog envelops. The tumult of the falls far warns mariners descended in canoes.’
And that is the first colonial record we have of Grand Falls in New Brunswick, where the St John Rover drops 23 metres into a sheer, winding, and it must be said gorgeous gorge.
It’s a fun historical tidbit. But I’m also including it to get you thinking about… time. 1686 was forever ago. Before the steam engine. Before the piano. Before even the flush toilet.
But as with Chaudierre Falls last time, Grand Falls and its gorge let us dive into geological time. Here we can find three stories told simultaneously - each on their own dizzying time scale, all of them making 1686 seem like five minutes ago.
The rocks at Grand Falls and Chaudierre Falls have a fundamental similarity. They’re both sedimentary, and were both built up on the bottom of the Iapetus Ocean as it began to close in the Ordovician Era.
So if that’s true… why do they look so different? We’ve got sharp bands of green and red strata at Chaudierre - and massive, lumpy dark grey formations with lines of white at Grand Falls.
Well, despite coming from the same source, they have one disparity that it turns out makes all the difference.
Chaudierre Falls is part of the Sillery Formation. Its sediments were laid down deep on the Iapetus seabed.
But Grand Falls is part of the White Head Formation. The marine environment that gave rise to these sedimentary rocks was much shallower.
And that’s it. So why would the depth of the water matter so much? Because shallow water means more life.
This is all limestone. Calcium Carbonate. And not all, but the vast majority of limestone on earth is formed from the compression of billions of corals and shells. This gorge is the sum total of millions and millions of generations of ancient marine life, squashed by geological processes into rock.
So again - the formations at Chaudierre Falls and Grand Falls are both sedimentary rocks formed in the same general area of the same ocean. And their folds and faults are both a record of the orogenies that gave birth to the Appalachians.
But just the relative depth of the ocean at each location as they formed was a cascade effect that led to the striking difference we see today.
Like pretty much every square metre of Canada, Grand Falls was impacted by the last ice age and the advance and retreat of glaciers.
In this case, the gorge itself owes its existence to the power of glacial meltwater blasting through what is now the St John River as the Laurentide Ice Sheet melted.
But that’s not all - there’s evidence that the ice age actually completely changed the path of the whole river - and then changed it back.
We’ve talked before about isostatic depression. In short: when enough ice is sitting on top of the land - like in an ice age - that land actually significantly sinks - like, up to 300m down.
And it can sink differentially depending on the kind of rock, thickness of the land, and how much ice cover it has. That all means that during and immediately after the ice age, there’s a totally different topography here, which would naturally change the course of the river as water seeks its level.
And we’ve also talked about isostatic rebound: once the ice melts, the land is free to bounce back to its old height. So that process diverted the St John River and then, eventually, reverted it to its present course where the gorge was carved.
Finally, Grand Falls Gorge has some great examples of a relatively recent geological anomaly.
Along the length of the gorge you’ll see these weirdly smooth, deep holes in the rock. They can look out of place or even intentionally carved. But they’re actually the natural outcome of some very simple, specific conditions in water flow and relentless erosion.
As water rushes over Grand Falls and beyond, the shape of features in the gorge will force it into eddies and whirlpools with a circular flow.
Those can do erosive work on their own but to get these potholes, you add one more tiny but necessary ingredient: pebbles. Big enough to be abrasive, small enough to get trapped in the churn of the water, harder than the surrounding rock.
When they’re caught in these little whirlpools they roll endlessly around the perimeter and slowly but surely wear down the rock deeper and deeper. And it’s a positive feedback loop. The deeper the pothole, the stronger its whirlpool and the more pebbles can get caught in it, reinforcing the effect. In Grand Falls Gorge that can result in potholes 9 metres deep.
Just like Chaudierre Falls, Grand Falls Geology can tell us simultaneous stories on massively different timescales - and show us how even ‘close by’ events, when viewed in geologic time, can result in massively different outcomes.
It may feel like the 350ish years between us and Mr Saint-Vallier is forever. But these potholes could have taken 10 times that long to be carved. The glaciers that diverted the St John river? 43 times further back. And the origins of the White Head Formation? One million, two hundred and eighty five thousand, seven hundred and fourteen times further back.
Time is, in short, crazy.