ABSTRACT: Evolution holocene d'un sector de la batture de Montmagny (Quebec): troncs d'arbes fossils, meandre relique et galets de quartz. [Holocene evolution of a section of the tidal flat at Montmagny (Quebec): exhumed tree logs, a relect meander, and banks of small gravel with 25% of quartz pebbles]

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At Montmagny, a wide (1-2 km) muddy and clayey tidal flat characterizes the shore zone. The section between Riviere du ud and Riviere du Vieux Moulin, which is a bird sanctuary, is a protected area. Thousands of geese use this rest area during the autumn hunting period. Since the creation of the Federal migrating bird sanctuary, geese activity resulted in severe vertical and lateral erosion of the tidal flat, particularly the bulrush (Scrirpus americanus) tidal marsh area. During the first half of the last decade, a few hundred tree logs, formerly buried into the muddy intertidal deposit overlying the clayey substrate, have been exposed to the surface. Among the identified species (75), the deciduous species account for 83% whereas the remaining 17% are resinous species. The main deciduous species are the sugar maple (Acer saccharum), the American beech (Fagus grandifolia), and birches (Betula spp.), whereas hemlock (Tsuga Canadensis) is the most common resinous species. The age of the tree logs ranges from 110+/-60 to 4540+/-100 years BP, but 44% are in the bracket 1400-2500 and 25% are 100 to 900 years BP. Two other characteristic features of the area are the occurrence of thin banks of small gravel containing about 25%of quartz pebbles, and a relict meander cut into the 6-m terrace adjacent to the tidal flat, suggesting the existence of a former stream during the Holocene and the subsequent lateral erosion of the low terrace. The evolution of the Montmagny tidal flat suggests variations of the mean sea level during the Holocene.

Bibliography of Canadian Geomorphology