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.

Transverse micro-erosion meter (TMEM) stations were installed in rock slabs from shore platforms in eastern Canada. The slabs were put into artificial sea water for 1, 6 or 11 hours, representing high, mid-and low tidal areas, respectively. The TMEMs were used to record changes in surface elevation as the rocks dried during the remainder of the 12 h of a semi-diurnal tidal cycle. A similar technique was used on the same rock types at intertidal TMEM stations in the field, as the rocks dried during low tide. Argillite and basalt surface contraction was from 0 to 0·04 mm: there was little surface expansion. Sandstones contracted by up to 0·03 mm in the field, but there was almost no contraction in the laboratory. Argillite and basalt contraction tended to be greatest in the upper intertidal zone, and to increase with rates of longer-term surface downwearing, but there was little relationship with rock hardness or air temperature and humidity. Changes in elevation at the same points at TMEM stations in the laboratory and field were quite consistent from one tidal cycle to the next, but there were considerable variations within single tidal cycles between different points within each station. The data suggest that contraction within the elevational zone that is normally submerged twice a day by the tides is by alternate wetting and drying. Short-term changes in elevation are generally low compared with annual rates of downwearing owing to erosion, but they may generate stresses that contribute to rock breakdown.drying on the shore platforms of eastern Canada, Trenhaile (2006) used a TMEM to measure the effect of intertidal elevation and air temperature and humidity on the surface expansion and contraction of drying slabs of basalt, sandstone and argillite. This work was conducted using de-ionized water in order to isolate the effect of wetting and drying by eliminating the effect of salt crystallization and other salt-related mechanisms. The present paper extends that research using three TMEMs to measure tidally induced rock surface contraction and swelling in artificial salt water in the laboratory and in sea water in the field. Short-term variations in surface elevation provide error terms for MEM and TMEM measurements, and they may generate stresses that contribute to surface downwearing by wetting and drying and salt weathering. The term 'downwearing' is used in this paper to refer to the permanent lowering of rock surfaces by erosion, whereas 'contraction' and 'expansion' are used for short-term changes (negative and positive, respectively) in surface elevation. Rock surface expansion and contractionFigure 3. Platform profiles and location and number of the TMEM stations used to measure short-term changes in surface elevation. Rock surface expansion and contraction 1383 Figure 4. Argillite surface changes in the laboratory. The two boxed numbers separated by a slash in figures 4-6 and 11-12 show the mean temperature and relative humidity of the air, respectively.Changes in surface elevation were generally a lit...

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

No abstract