Mount Royal, A Territory to Discover

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The Tic-tac-toe Quarry (Côte-des-Neiges and Belvedere Roads)
 

There are no longer any active quarries on Mount Royal, but you can still see signs of exploitation of rock used for construction. The wall reveals layers of grey rock in alternating shades of light and dark. These are Trenton limestone strata, formed about 450 million years ago. The layers are cut through with sheets of magmatic rock, called dikes, formed when magma pushed up through the limestone, about 125 million years ago.
Dikes were formed one after the other through reoccurring intrusions of magma. The most spectacular intersection of dikes here is one that looks like a giant game of tic-tac-toe…
 


There are no longer any active quarries on Mount Royal, but you can still see signs of exploitation of rock used for construction. The wall reveals layers of grey rock in alternating shades of light and dark. These are Trenton limestone strata, formed about 450 million years ago. The layers are cut through with sheets of magmatic rock, called dikes, formed when magma pushed up through the limestone, about 125 million years ago.
Dikes were formed one after the other through reoccurring intrusions of magma. The most spectacular intersection of dikes here is one that looks like a giant game of tic-tac-toe…
 

Credit:

Tic-tac-toe dike
Les amis de la montagne Collection


 


Questions :

(1) Mount Royal is an ancient volcano.

Answer False

(2) The First Nations' people made tools and weapons out of stone on Mount Royal.

Answer True

Fossilised crinoïdes

Dyke

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Mission geology

A new mission: retracing the geological history of Mount Royal.
A magnifying glass, a flask of chlorhydric acid, a sketch pad, and above all, a good geology book. Okay, I'm ready.

At the foot of the mountain, at the Peel entrance, a beautiful rock face. You can clearly see the almost horizontal layers, or stratum.
A drop of acid: bubbles.This rock fizzes; it must be limestone.
There! A little white spot. It looks like a fossil. Yes, it's the stem of a fossilized crinoid. An animal that lived over four hundred million years ago in tropical seas.
Hmm, strange…These rocks must have been formed about four hundred million years ago in a warm sea. A first clue.

I walk up Olmsted road. After the fountain, some more small limestone walls.
But, there’s something that doesn't fit. It's weird. Here and there, very different bands of rock cut across the layers. When I test them with the acid, there's no reaction. Hmm.They're dykes, veins of intrusive rock.
So the sedimentary layers were really put through the wringer.

I climb the large staircase towards the Chalet. At the top of the staircase, a small rocky outcrop. The rock is a rusty color on the surface and greyish black underneath. Very fractured. When I break it, the edges are sharp. There is no reaction to the acid.
It’s hornfels, a metamorphic rock formed under great pressure and high temperatures…
Something definitely shook up these old sedimentary rocks.

Let’s continue climbing. In the picnic area behind the Chalet, worn, rounded rocks crop up through the grass. No reaction to the acid. With my magnifying glass, I can see large black minerals. And the rock is shot through with many veins. This time, it looks like igneous rock. Hmm, yes, it’s gabbro.
So in just one hour, I’ve seen three families of rock: sedementary, igneous and metamorphic. Well, Mount Royal’s formation was decidedly not a straightforward undertaking.
I think I’ll continue my investigation at Smith House. In the basement, what a surprise: I’ve found the heart of the mountain. One more clue.
I think I can establish an initial scenario.To sum up: over four hundred million years ago, sedementary rock formed at the bottom of a sea, close to the tropics.
Then, magma flowed up from the depths, under the sedementary layers, but did not break through. There was therefore no volcano. The magma cooled below the earth’s crust, and was transformed into a very hard rock, called gabbro.But the mountain did not yet exist.
The layers that covered this core of hard rock gradually eroded. It was wind, rain and above all, the glaciers, that fashioned Mount Royal.

So that’s that. I’m sure other hypotheses should be verified, but I think that I have fulfilled my mission.