The Montreal region is known for the many islands that dot the St. Lawrence River, the Rivière des Prairies, and the Rivière des Mille Îles. But did you know that Montreal, Jésus, and Perrot islands, the largest in the metropolitan area, gradually rose out of the water as the land rebounded after the invasion of the Champlain Sea?
Different processes give rise to different shapes
A complex network of very ancient faults, the varied resistance to weathering of different rock types, and the vagaries of fluvial and glacial erosion all helped form the distinctive Montreal archipelago. The islands that are part of this group are easily recognizable by their irregular, often pebbly shorelines. In many cases, riprap and even walls have been installed to protect the shorelines of these islands from erosion by currents and waves. Spring ice can also cause significant damage, even to the most solid of protective structures.
Approximately 11 500 years ago, the tops of the Monteregian Hills emerged from the Champlain Sea. Then, as the continent gradually rebounded, the islands formed by the hills grew larger and larger.
Other islands were formed by the deposition of unconsolidated materials in the Champlain Sea and in the lake left behind as it receded. The Boucherville Islands, and other islands of this type, are more regularly shaped and are elongated in the direction of the current. Although their relatively flat tops are sandy, they are composed primarily of clay. They are more prone to erosion, and the small cliffs along their edges tend to collapse during spring floods.
About 9000 years ago, the development of the first channels in the clays marked the transition from a marine to an estuarine environment. The rivers emptied into a body of water that was shrinking in size.
Finally, a third type of island should be mentioned. These man-made islands were created from materials excavated during the construction of the Montreal subway. Visitors to La Ronde or the Casino de Montreal do not always realize that they are treading on artificial ground.
Today, the islands and a few shallow lakes, including Lac des Deux Montagnes, Lake Saint-Louis,and Lake Saint-Pierre, are an integral part of the landscape of the Montreal region.
Champlain Sea Retreat Animated view (60 s - slow motion) [GIF, 790.7 kb, 500 X 500]
A Satellite View of Montreal
Satellite imagery helps us interpret the landscape. The high areas of the Laurentians and the Monteregian Hills appear red, whereas the lowlands or terraces, which are at lower altitudes, are depicted in ochre, green, and blue.