By: David Eden
To dispel any alarm, I should clarify that the Iceberg Alert applied about 50,000 years ago and is not a current concern. Unfortunately, written records of the pre-historic Iceberg Alert have not survived more recent glaciations in the Toronto area. However, there is a record of one of the larger icebergs preserved in the muddy and sandy sediments of the Scarborough Bluffs. This geologic structure was the main subject of my M.Sc. thesis at the University of Toronto: "Ice Scouring as a Geologic Agent: Pleistocene Examples from Scarborough Bluffs and a Numerical Model". Along with my M.Sc. supervisor, Dr. Nick Eyles, I interpret this structure to be an ice scour caused by a floating iceberg in an ancestral ice-dammed Lake Ontario.
Ice scours are the track left behind on the sea floor or lake floor when an iceberg, or a mass of seasonal pack ice, "bottoms out" in shallower water. Ice scours criss-cross the sea floor in modern polar oceans, in areas such as the Beaufort Sea and the continental shelves off Greenland, Norway and Antarctica. Closer to home, modern ice scours from seasonal pack ice occur on the floor of Lake Erie. Ancient examples are preserved in the Glacial Lake Agassiz sediments in Manitoba (featured in WAT ON EARTH Vol.10 (2) p.4, 1997) and show up clearly on aerial photographs. Chris Woodworth-Lynas, then with C-CORE in Newfoundland, studied the Lake Agassiz scours in cross-section with the help of a backhoe.
Through shoreline erosion and slope failure, nature has graciously exposed a cross-sectional view of an ice scour at Cudia Park on the Scarborough Bluffs. This is fortunate, because the rappelling equipment I needed to access the scour was not rated for heavy construction machinery. Photographs and a sketch of the ice scour structure are shown in Figures 1 and 2. The structure is cut in clayey silt and is infilled with storm-deposited sands. The scour trough is about 4 m deep and 10 m wide, and has sub-scour sediment deformation up to 5 m below the bottom of the scour trough. A ground penetrating radar (GPR) survey confirmed that the scour extends further inland. Using a numerical model of the ice scour forces, we estimated that the downward scouring force of the ice mass was about 5 MN, which is consistent with an ice keel draft of about 20 m. This water depth estimate is consistent with independent sedimentological estimates based on the amplitude of nearby sand beds. We have published papers on the study in Sedimentology and the Canadian Geotechnical Journal.
I conducted the M.Sc. studies part-time while working for Golder Associates in Mississauga, who provided partial support for the programme. In addition to thanking Dr. Nick Eyles for excellent supervision and helpful insight, I'd like to thank Mike Doughty and Kyle Hodder of the University of Toronto for assistance in the research.
Figure 1 (above): Photographs of Cudia Park Ice scour. A: summer view, B: winter view. arrows in B indicate "berms"of ploughed sediment at the sides of the scour and deformation below the scour.
Figure 2 (above): Sketch of ice scour shown in Figure 1, showing stratigraphy and sediment facies types.
David Eden is a Waterloo grad (B.A.Sc., Geological Engineering, 1996) who recently made a career switch to risk management at Ontario Power Generation. He is a part-time MBA student at the Rotman School of Business, University of Toronto.