On September 19, 1889, part of the rocky promontory above rue Champlain at the west end of terrasse Dufferin collapsed, crushing seven houses. The toll: 35 deaths, numerous injuries, and significant property damage. This part of the Québec promontory, better known as 'cap Diamant', was one of the most dangerous inhabited areas of the region, where rockslides claimed at least 85 victims during the nineteenth century. The area around rue du Petit-Champlain has also seen some dramatic rockslides, the most memorable of which took place in 1841 and 1889.
The promontory was formed by the interplay of tectonic uplift and erosion that created the Appalachians. As a result, the layers of sedimentary rock that make up this feature now dip parallel to the slope, and in places are even vertically inclined. As a result, gravity is able to cause some layers to slide along other layers. Rockslides often occur after heavy rains, when water infiltrates the crevices, or during periods of successive freezing and thawing.
The Montmorency River is known for more than its famous falls. It is also renowned for the spectacular ice jams that regularly form along its length. During a winter or spring thaw, the ice cover breaks up into blocks that are transported by the current and pile up where the river narrows. In this way, a temporary dam can form, causing the water to rise and upstream shoreline areas to flood. This hummocky ice can sometimes reach heights of 5 to 7 m, or twice that of a house. Occasionally, an ice dam breaks suddenly under the pressure of the water behind it, sending an outburst flood surging downstream to inundate the lower reaches.
Spring ice dams on the Montmorency river, Île Enchanteresse (Courtesy of MRNQ)
Water-front residences of Île Enchanteresse threatened by the ice flow on the Montmorency river (Courtesy of MRNQ)