The composition and geological origin of mount Saint-Bruno is similar to that of the other Monteregian hills. Formed by an intrusion of magma in the earth's crust associated with a hotspot created by the opening of the atlantic ocean approximately 124 million years ago, the Monteregian hills have been exposed to the elements by the progressive erosion of their paleozoic sedimentary layers, which are softer than their core of metamorphic and igneous rocks that typically form plutons. Contrary to popular belief, they are not volcanoes.