Contrary to popular belief, Mount Royal is not an ancient volcano that is now extinct. This misconception has arisen because of the shape of the hill. Mount Royal and some other Monteregian Hills have central depressions, many of which are occupied by a lake such as Lac des Castors on Mount Royal and Hertel Lake on Mount Saint-Hilaire. But these lakes are not crater lakes, and the rock that forms these hills is not volcanic!

One hundred and twenty-five million years ago, magma from depths of several tens of kilometres started to move toward the surface along fracture zones in the rock. As it cooled and crystallized, its ascent toward the surface was halted.

When they solidified, the igneous rocks were still several kilometres below the surface of the Earth. They were gradually unearthed by the slow, natural process of erosion, at an average rate of 4 cm/millennium. The hills now dominate the landscape because they are far more resistant to erosion than the rocks that surround them. Today, their intriguing alignment breaks the monotony of the landscape in the St. Lawrence Plains.

The quarry at Mount Saint-Hilaire is a mineralogical site known throughout the world for the abundance and diversity of the minerals found there. Montreal, too, lays claim to a number of small unique finds, including montroyalite, a mineral named for the city's own hill. Quarries and excavations are havens for mineral collectors and, in Montreal, the Francon Quarry is the best known locality for collecting rare minerals.

The hills at Oka, Rigaud, and Saint-André-Est are small, distinctly older bodies of rock that formed long before the Monteregian Hills. It is important to protect the integrity of the habitats that cover the rock bodies and hills in the Montreal region because they do much to maintain the biodiversity of the area.