No abstract

No abstract

Downwearing rates were measured on shore platforms at about 200 transverse micro-erosion meter (TMEM) stations, over periods ranging from 2 to 6 years. There were seven study areas in eastern Canada. The platforms were surveyed and a Schmidt Rock Test Hammer was used to measure rock hardness. More than 1200 rock samples from three of the study areas were also subjected each day, over a 3 year period, to two tidal cycles of immersion and exposure, which simulated the central intertidal zone. A further 840 samples were subjected to longer periods of exposure and immersion, over a 1 year period, which represented different elevations within the upper and lower intertidal zone, respectively. These experiments suggested that tidally generated weathering and debris removal is an effective erosional mechanism, particularly at the elevation of the lowest high tides. In the fi eld, mean rates of downwearing for each study area ranged from 0·24 mm yr −1 to more than 1·5 mm yr . Rates tended to increase with elevation in the fi eld, with maxima in the upper intertidal zone. This trend in the fi eld cannot be attributed entirely to the tidally induced weathering processes that were simulated in the laboratory, and must refl ect, in part, the effect of waves, frost, ice, and other mechanisms. It is concluded that there are no strong spatial downwearing patterns on shore platforms, and that downwearing rates in the intertidal zone are the result of a number of erosional mechanisms with different elevationeffi cacy characteristics. Furthermore, even if only one or two mechanisms were dominant in an area, any resulting relationship between downwearing rates and elevation would be obscured or eliminated by the effect of variations in the chemical and physical characteristics of the rocks.

No abstract

This research is conducted on a mesotidal, argillite shore platform at Mont Louis in Gaspé, Québec, and on macrotidal platforms in the basalts of Scots Bay and the sandstones of Burncoat Head in Nova Scotia. Rock samples have been subjected to wetting and drying and to salt weathering cycles. The platforms were surveyed; rock hardness was determined with a Rock Test Hammer; waves were recorded in the field; and downwearing rates were measured at 56 micro-erosion meter stations over 1 to 3 years. Weathering is the dominant process at Mont Louis, although the horizontal platform may have been cut by waves at the high tidal level. Wave backwearing was much more important than downwearing by weathering during the Holocene at Scots Bay. Wave quarrying only occurs on a few scarps today, however, and without much abrasive material, slow downwearing now dominates over most of the platform surface. Waves probably help to remove loosened sand grains at Burncoat Head, thereby contributing to platform downwearing. Abrasion is also important in places, but the data suggest that backwearing by wave quarrying and probably frost has been a little more important than downwearing by abrasion and weathering during the Holocene.Processus des plates-formes littorales de l’est du Canada. Une plate-forme littorale d’argilites soumise à des marées modérées à Mont Louis en Gaspésie, Québec, et des plates-formes soumises à des fortes marées dans les basaltes de Scots Bay et les grès de Burncoat Head de la Nouvelle-Écosse furent étudiées. Des échantillons de roches ont été soumis à des cycles de mouillage et de séchage et d’haloclastie. Le profil des plates-formes a été mesuré, la dureté des roches a été établie par le test du marteau de Schmidt, les vagues ont été mesurées sur le terrain et le taux d’usure vertical a été quantifié à 56 stations avec des appareils détectant la micro-­érosion, le tout sur une période d’un à trois ans. L’altération est le processus dominant à Mont Louis, bien que la plate-forme horizontale ait été entaillée par les vagues au maximum des marées hautes. L’usure horizontale par les vagues était plus importante que l’usure verticale par altération durant l’Holocène à Scots Bay. L’extraction de grands blocs par les vagues ne se produit toutefois de nos jours que sur quelques abrupts et, en l’absence de matériel abrasif, une lente usure verticale domine maintenant sur la plupart des plates-formes. Les vagues ont probablement aidé à éliminer les grains de sable produits par l’altération à Burncoat Head, contribuant ainsi à l’usure verticale de la plate-forme. L’abrasion est également importante à certains endroits, mais les données indiquent que l’usure horizontale par les vagues, et aussi par le gel, a été un peu plus importante que l’usure verticale par l’abrasion et l’altération durant l’Holocène