Date of Award


Publication Type

Doctoral Thesis

Degree Name



Earth and Environmental Sciences


Health and environmental sciences, Earth sciences, Downwearing, Intertidal zone; Shoreline erosion, Weathering


Alan Trenhaile




There has been a debate, for more than a century, on the precise modes of development of shore platforms, and on the relative and absolute importance of marine and subaerial processes in micro- to megatidal environments. There are few reliable data, however, to assess their contributions to shore platform formation and evolution. This study is concerned with the acquisition and analysis of laboratory and field data on rates of surface downwearing (erosion in the vertical plane), primarily by weathering, on shore platforms in eastern Canada. Tidal simulators have been used to study the breakdown of rocks in de-ionized water and in artificial sea water. The simulators consisted of series of basins, reservoirs, pumps and timers. Each basin contained rock cores and, according to the period of inundation or exposure, represented the high, mid- or low tidal levels. More than one-thousand rock samples (sandstone, basalt and argillite) from eastern Canada were exposed to the experimental conditions for a period of 3 years (2190 tidal cycles). There was considerable variation in downwearing rates within each treatment group, and mean rates for basalts (1.4 mm yr1) and argillites (2.8 mm yr1) were highest in de-ionized water and for sandstones (1.8 mm yr1) in salt water. Mean downwearing rates were generally highest in the mid- to high tidal zones, and declined with increasing elevation. The relationship between downwearing rates and elevation was more complex in the field. Downwearing in the upper portions of the platforms cannot be entirely attributed to tidally induced weathering processes, but must also reflect the work of abrasion, wave quarrying, and other erosional mechanisms. The experimental results are broadly consistent with the data obtained from over 200 traversing micro-erosion meter stations installed along shore platform profiles in the field. Downwearing rates in Gaspe support the hypothesis that the platform was cut largely by mechanical wave erosion during a period of higher relative sea level during the Holocene, and then lowered by weathering to its present elevation as sea level fell. In the megatidal Bay of Fundy, mechanical wave erosion is still an important mechanism, although weathering is also important and it is dominant on platforms that lack seaward facing scarps or upstanding rock strata.