Abstract

Extra-sensitive or quick clays are characteristic deposits of the Champlain Sea, which covered the St. Lawrence River valley about 11 000 years ago. Landslides of the earthflow type occur along many of the stream valleys. Rapid dissection and high precipitation are commonly associated with earthflows.Landforms characteristic of earthflows include bowls, slump slice ridges, pinnacles, inter-bowl ridges, and ribbing. Large earthflow complexes cover areas of several square miles, particularly just south of the St. Narcisse moraine. Earthflow debris spreads over alluvial deposits, many of which contain datable plant matter. Only a few prehistoric earthflows have been radiocarbon dated, but such data hold promise for the study of alluvial history and earthflow occurrence. At least two consecutive phases of earthflow activity have been noted in some areas, and a time span of several thousand years appears to be involved.Earthflows tend to occur in groups; historic flows in the area, including the classic St. Thuribe slide, have occurred in close proximity to older slides. Therefore certain slide-prone areas can be designated for planning purposes. It is advisable to avoid having building sites near present stream valleys.

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