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TitleGeorgian Bay bedrock erosion: evidence for regional floods, Ontario
LicencePlease note the adoption of the Open Government Licence - Canada supersedes any previous licences.
AuthorSharpe, D RORCID logo; Leduc, G; Smart, C S; Shaw, JORCID logo
SourceGeological Survey of Canada, Open File 8909, 2023, 77 pages, Open Access logo Open Access
PublisherNatural Resources Canada
Documentopen file
Mediadigital; on-line
File formatpdf
NTS30M; 31D; 31E; 31L; 40O; 40P; 41A; 41B; 41G; 41H; 41I; 41J; 41K
AreaGeorgian Bay
Lat/Long WENS -84.5236 -79.0786 46.6397 43.5383
SubjectsScience and Technology; general geology; Nature and Environment; bedrock geology; erosion; floods; geological surveys
Illustrationslocation maps; illustrations; tables; satellite imagery; photographs; schematic models
ProgramGroundwater Geoscience Archetypal Aquifers of Canada
Released2023 03 14
AbstractWe provide an updated presentation of the spectacular erosion forms at French River Ontario (Kor et al. 1991) based on new methods of data collection and wider observations. This work includes ~ 10 more detailed study sites, documentation of the range of forms over a larger area, the use of extensive drone image capture and ground surveys, as well as a detailed inventory of forms. Key sites are illustrated using video images. The update extends the conclusions of the Kor paper regarding the significance and scale of subglacial meltwater erosion with some novel findings. We document the importance of plucking (including hydraulic plucking) and, the control of structure on s-forms, which were not highlighted in the Kor study. Apparent cavitation erosion forms are prominent across the study area and provide support for inferred high-velocity meltwater flow. A growing interpretative framework includes discussion of evidence to test a theorized hydraulic sequence of sheet-channel-distributed flow, followed by re-grounding of glacial ice as meltwater flow waned. This hydraulic sequence may also be complementary to observations in thick sediment terrain down flow.
Summary(Plain Language Summary, not published)
It is assumed that most erosion during glacial periods was due to glacial abrasion as ice crept across the landscape. Field studies (including stunning drone images) of spectacular rock erosion forms at French River, Georgian Bay, Ontario reveal the role of exceptional subglacial meltwater floods in sculpting the landscape. Early sketches of the Georgian Bay rock forms by Andrews (1883) captured the essential science issue: the forms were produced by turbulent eddies in the flow. In contrast, the glacier, which most geoscientists have assumed was the agent of erosion, flows as a ‘block’ with no eddies that could explain the abundant presence of potholes in the eroded rock. The Georgian Bay floods were ~ 100 km across, 10s of metres deep that travelled at 5-10 m/ sec (faster than most modern rivers), scoured the landscape of any sediment and eroded channels on the floor of Georgian Bay. The floors of the channel rise down flow, indicating that they formed under high (subglacial) pressure.

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