Oh, Canada! Part 2: Impact

Intro: Oh, Canada!

Part 1: Oh, Canada! Part 1: Monadnocks

The drive through the Charlevoix, the area on the northern bank of the St. Lawrence from Quebec City to la Malbaie, is jaw-dropping and quintessentially Canadian. Anti-moose fences running along the sides of the highway underline fantastic views of the Laurentian mountains where my cousins learned to ski. When the road winds through enough civilization for the moose fences to disappear, the civilization takes the form of adorable clapboard houses with brightly painted accents and laundry drying on the line. The entire landscape looks like it jumped out of a picture book.

charlevoix 1

However, around 342 million years ago was a really terrible span of time to live there.

It’s hard to tell why from the ground. The road from Quebec City dipped down steeply to get to the sleepy resort town of Baie-St.-Paul, climbed up a winding local road to the village of Les Eboulements perched on a cliff, and then wound back downhill to La Malbaie further north. To get to the Quatre Vents Garden, we drove uphill again along the coast north of La Malbaie. Looking at the map, the main highway that we didn’t take sweeps around in a half-circle from St. Paul to La Malbaie following the easy route through the valley.

That’s pretty odd. The rest of the valleys in the area run roughly perpendicular to the coast. Why would a valley do a u-turn and meet the coast on both ends? (valley represented by the white line on the map below)

Charlevoix map

Well, it turns out that this particular valley had some help in the form of a 1.2 mile-wide chunk of rock.

The meteor crashed into what is now the Charlevoix in the early Carboniferous period; scientists’ current best guess is that it struck 342 million year ago with a 15-million year range of error on either side. The impact razed any flora and fauna, flattened the landscape, and created dramatic shatter cone structures in the whatever underlying Ordovician limestone it didn’t pulverize completely.

https://i2.wp.com/www.impact-structures.com/wp-content/uploads/2014/11/3.png

Image of an excavated shatter cone, courtesy of impact-structures.com

 

Initially, the crater was as bowl-shaped as anyone would intuitively expect from bedrock that had been punched by a giant boulder from outer space. Over time, however, the center of the crater rose as a reaction to the absence of pressure after the impact. That rebound created the higher elevation where Les Eboulements stands today.

Us humans have this impact to thank for the half-circle of relatively flat terrain between Baie-St.-Paul and La Malbaie, where 70% of the population of the Charlevoix region now resides.

https://i2.wp.com/www.impact-structures.com/wp-content/uploads/2014/11/radarbild-Charlevoix.jpg

The crater has a much more uniform and gentle gradient, compared to the rugged highlands of the Laurentian Highlands. A RadarSat image of the Charlevoix area, courtesy of the Canadian Space Agency via impact-stuctures.com

But where is the other half of the crater? It was no match for the seismic and glacial stresses of the past 340 million years. Pangaea crashed together 40 million years later and was torn apart 125 million years after that, and between 2.5 and 0.7 million years ago an ice cap took advantage of those faults in the crust to carve away the weak points. When that ice cap shrank, all that glacial melt poured through the St. Lawrence and washed away what was left.

Charlevoix from Quatre vents

The view looking south from Quatre Vents Garden on the northern rim of the crater, across La Malbaie in the sunken impact ring, towards the rebounded area of Les Eboulements.

I wish I had discovered Observatoire de l’Astrobleme de Charlevoix and its awesome geological app before the trip, but if I ever go back I’ll definitely get it!

Up Next: Oh, Canada! Part 3: Clay and Quakes

 

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2 thoughts on “Oh, Canada! Part 2: Impact

  1. Pingback: Oh, Canada! Part 3: Clay and Quakes | Blue Marble Earth

  2. Pingback: Oh, Canada! Part 1: Monadnocks | Blue Marble Earth

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