Canadian Rockhound: How Mont Saint-Hilaire was Formed

How Mont Saint-Hilaire
was Formed
By Martin van Kuilenburg, edited by Dirk Schmid

As you look for that prize mineral specimen at Mont Saint-Hilaire, did you ever notice the mountain beneath your feet and wondered how it got there? The geology of Mont Saint-Hilaire is quite interesting, but too complex and lengthy to discuss here. However, there are several excellent sources available that describe the geology and mineralogy in detail, and are highly recommended for further reading. A list of these sources appear elsewhere in this issue of the Canadian Rockhound. In this article we will briefly summarize the geological processes and events that resulted in the formation of the mountain we see today.

Geological Profile
Fig 1. Simple diagram showing the extent the magmas travelled upwards towards the surface. Today, the portion of these magmas exposed at the surface form Mont Saint-Hilaire.

More than 130 million years ago, the area around Mont Saint-Hilaire looked very different. A large body of water covered the land. The sea bottom was flat and over millions of years many layers of sediments were slowly deposited.

Tectonic forces slowly caused the land to rise. Then, during the Cretaceous period, between 120 to 135 million years ago, magma from a depth of more than 115 kilometres started to move toward the surface (Fig 1). On the way up, many different minerals from the surrounding rocks were dissolved. Following cracks and fissures in the rocks, the magma formed a pool of molten rock at a depth of 15 km. There the various minerals started to separate into different solutions.

Following other cracks, these magmas approached the surface, but before they emerged they solidified into massive intrusions deep underground. There they slowly cooled over millions of years.

Meanwhile the forces of erosion were also slowly wearing down the surrounding sedimentary rocks. When the magma intrusions were reached they eroded much slower, since they were made of harder rock. Today Mont Saint-Hilaire sticks out 375 meters above the surrounding land.

A total of 10 similar mountains were formed in this way. Known collectively as the Monteregian hills, they are: Oka, Mont-Royal, Saint-Bruno, Saint-Hilaire, Rougemont, Saint-Gregoire, Yamaska, Shefford, Brome, and Megantic.

Mont Saint-Hilaire is today known worldwide for its fine mineral specimens, including those that remind us of their origin: hilairite and monteregianite. The mountain also produces aggregates used in the manufacture of concrete and in the foundations of our roads.

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The preceding article was first published in the June 1998 issue of Geminews, the official newsletter of the Montreal Gem & Mineral Club. Reprinted in the Canadian Rockhound with permission from the author.