Reading Rocks - The Bay Of Fundy’s Sculpted Sandstone
All along the Bay of Fundy, you’ll find outcrops of beautiful, naturally sculpted red sandstone. Where did it come from? Answering that question will take us back 250 million years and tell the story of how the Bay of Fundy itself was formed.
EPISODE NOTES
TRANSCRIPT
If you’ve been following our whole journey along the east coast, this is a familiar sight. We’ve visited the red stone of the Bay of Fundy before, investigating the secretive lives of mud piddocks, one of the rarest animals in Canada, who burrow into this very specific rock.
And in doing so, they’re burrowing into 250 million years of history. At the time I just tossed it off as ‘Triassic sandstone’. Now we’re gonna find out exactly what that means, why it’s here, and where that beautiful colour comes from. Because the story of these rocks is actually the story of the Bay of Fundy itself: how it got here and, how young it actually is.
So: 250 million years ago. The supercontinent Pangea, the last time most of the landmasses on earth will be stuck together… is just starting to break apart. More locally, that means that the North American and African plates are separating.
In their wake, they’re going to leave a rift valley. This, many many millions of years later, is what will become the Bay of Fundy. But it has a long way to go. And for now, it’s entirely landlocked: a complex mix of river systems, flood plains and intermittent lakes.
At the same time, the ancient Appalachians are experiencing a period of aggressive erosion. They used to be much taller - up to around 6000m, putting them on the same scale as the Himalayas. Today, the tallest Appalachian peak is Mount Mitchell in North Carolina, at 2037 meters - just to give you an idea of how intense this erosion has been.
And the Triassic, from around 250 to 200 million years ago, is a real great time for erosion. It’s hot and dry, limiting plant growth and making the soil unstable.
That said, when rain does come, it’s in massive deluges which occasionally, aggressively wear down the Appalachians.
And it turns out that the same conditions that make the Triassic so erosive make that eroded rock more susceptible to oxidization.
Chemically, oxidization transforms ferrous iron in rock to ferric iron. The reaction looks like this:
4 Fe2+ + O2 + 2 H2O → 4 Fe3+ + 4 OH-
And what it looks like is rust - iron oxide coats the rock or sediment grains and takes them from green or grey to orange or red.
So how exactly did Triassic conditions encourage it?
Well, higher temperatures just generally encourage chemical processes - like oxidization.
Plus, oxidization needs… oxygen, it’s right there in the name. In humid conditions, water can actually form a protective film around rock that slows or prevents oxidization. Not so much in the dry, dry Triassic. And in fact, the cycles of extreme drought and deluge are the best possible conditions for encouraging oxidization.
So we have our two key ingredients here: an era that encourages both rapid oxidization and erosion, turning those soaring mountains into sediments that flow into the newly formed rift valley.
There, they build up into distinct, thick layers thanks to the alternating periods of drought and deluge over millions of years and are compressed into sedimentary rock: sandstone.
But that’s only half the story. All we have now is a vast floodplain with layers of oxidized sandstone hidden under the surface. To get the sculptural formations of the Bay of Fundy - not to mention the bay itself - we need to call on our old friends the glaciers.
See, the Bay of Fundy has only actually been a bay for the last ten thousand years or so. It took the melting of the great Laurentide Ice Sheet to raise sea levels enough that our rift valley is flooded by the Atlantic ocean, at last becoming the bay we know today.
And even then, you don’t get this without one last bout of aggressive erosion, this time powered by the unique geography of the bay. As we talked about at the start of this east coast series, the Bay of Fundy has a funnelling effect on water combined with a synchronous tidal resonance that give it the highest tides in the world - up to 16 meters.
Those tides ceaselessly walloping the shore have carved back the sandstone - and done so differentially. Remember that these sediments were originally laid down in broad, distinct layers. Conditions when each one occurred produced harder or softer rock, now eroding at different rates and creating these flowing, curved features along with the colour variations that can produce banding in the rock as well.
Just one more example of how, behind every eye-catching feature of geology, hundreds of millions of years of history is hiding in plain sight.