Contrary to popular opinion and despite its shape, Mont St. Hilaire did not originate as a volcano. The mountain was formed instead as a series of plutonic intrusions into the sedimentary bedrocks of our region, that is, through the upwelling of magma that never managed to break the surface as a volcano but instead cooled slowly as a mass of igneous rock surrounded by the sedimentary rock it had melted and forced aside. The details of this geologic history of Mont St. Hilaire are fascinating and unusual.

During the Cretaceous era, about 130 million years ago, magma from deep in the earth began to well up and displace the crustal rocks already in place in our region. The magma rose to within 2,000 metres of the surface, and then gradually cooled to form a strong core of igneous rock. Over millions of years the softer sedimentary rocks above and around this igneous rock mass were gradually eroded away. Eventually, the mass of igneous rock was revealed at the surface. The sedimentary rocks are about four times less durable than the igneous rock, so as erosion continued the igneous rock mass began to stand higher and higher above the surrounding plain.

The sedimentary rocks at the surface in our region today were formed during the Ordovician era, about 500 million years ago. At that time 70% of what would eventually become North America was deep below the ocean surface. The seafloor sediments accumulating in this period eventually formed the bedrock of our region; as more and more sediment accumulated the deeper strata were compressed and heated sufficiently to harden into rock. Seafloor sediments continued to be deposited on this part of what would become North America throughout the Silurian and the Devonian eras, until about 360 million years ago. Finally, the land surface rose above the sea level and the erosion of these sedimentary rock layers began. When the magma intruded into our region during the Cretaceous era, it penetrated sedimentary rocks from the Ordovician, Silurian and Devonian eras. The Silurian and Devonian sedimentary rocks are now almost entirely eroded from our region, so that Mont St. Hilaire is a Cretaceous-era intrusion standing above mostly Ordovician-era sedimentary bedrock.

The magmatic intrusions that formed Mont St. Hilaire were probably triggered by the beginning of the formation of the Atlantic Ocean during the Jurassic era, about 160 million years ago. The tectonic forces associated with the formation of the mid-Atlantic rift appear to have created a hot spot under our part of North America. The hot spot itself is relatively stationary deep in the earth, but as North America was slowly pushed westward by magma welling up at the mid-Atlantic rift the occasional upwellings of magma formed the line of Monteregian Hills. Today the easternmost (hence youngest) of the igneous mountains formed along this line is Mont Megantic, near the Quebec border with Maine. There are, however, also traces of similar igneous plugs in the White Mountains of New Hampshire and out onto the floor of the Atlantic Ocean.

Mont St. Hilaire itself arose in three distinct intrusions, each with somewhat different characteristics in magma and contact with sedimentary rocks. The mixing of chemicals from the melting sedimentary rocks and the magma itself, plus the slow cooling of the magma far below the surface, contributes to the extraordinary mineral richness of the mountain.