Iceberg scour marks

Saturday, May 24, 1997

In the last issue of WAT ON EARTH I described ice-wedge polygons, - relict features left over from a short-lived regime of permafrost in southern Ontario in the period immediately after the ice retreat. As I explained, these features are virtually invisible at ground level but they can be clearly seen (under the right circumstances) from the air. The recent widespread media coverage of the floods affecting the southern part of Manitoba brought back some memories of traversing this region in the past, and it reminded me that other strange features can frequently be observed from the air.

In the Editorial for this issue I mentioned that a large part of Manitoba, as well as adjacent parts of northeast North Dakota, northwest Minnesota, northwest Ontario, and eastern Saskatchewan were covered by various stages of a huge proglacial lake, known as Lake Agassiz. The dimensions of this former lake are enormous. It was several hundred kilometres wide at the southern edge, at least 600 km wide halfway up present Lake Winnipeg and at least 500 km long through much of its life. It is difficult to synthesise the history of this complex lake, because the dimensions of Agassiz constantly changed. Ice still-stands raised water levels while ice-retreats uncovered new outlets and dropped water levels. Furthermore, the lake was time-transgressive as it followed the northward retreating ice; starting over 12,000 years ago in the extreme south and finally vanishing about 7,500 years ago close to Hudson Bay in the north. For those interested, there is a comprehensive book on Glacial Lake Agassiz, published by the Geological Association of Canada in 1983. Suffice to say that the presence of Lake Agassiz in the recent geological past is reflected in the extremely flat topography of southern Manitoba, and in the richness of the alluvial clays which support the extensive agriculture of the region.

In May 1990 I was flying back from a meeting in western Canada. As the aircraft crossed the area between Lake Manitoba and Lake Winnipeg I started to see that large areas of the landscape were criss-crossed with straight and curved lines. The features gradually became more and more pronounced, and I became more and more excited. Eventually I pulled out my camera (which I normally keep with me on all flights) and began clicking off photographs. This attracted the attention of the person sitting next to me who asked what it was that I found so interesting. I replied that the ground beneath the aircraft was covered with iceberg scour marks. He paused for a few seconds and then said that he was in the Canadian coastguard and that there was no way that there were iceberg marks below an aircraft in the middle of the prairies! I explained that these were relict scour marks caused by the keels of Ôbergs being pushed by winds into the shallow waters of a former glacial lake which occupied this region 10,000 years ago. To cut a long story short, I eventually ended up standing on the seat alternating between photo-taking and "lecturing" to 6 rows of passengers sitting behind me on the north side of the aircraft!

This was not the first time I had seen these features. I think the first time that I was conscious of them was returning from a field trip in northern Manitoba in May 1982. As the aircraft approached Winnipeg from the north and started to reduce altitude for landing these same features had appeared beneath the wing. The scour marks are anywhere from several hundred metres to 5 or more kilometres in length. The Lake Agassiz basin is not the only place that they can be seen (for example, they can be seen in the extreme southwest of Ontario between Tilbury and Leamington [Morris and Kelly 1997]), and there are probably many other locations in different areas where large proglacial lakes existed in the past. So, donÕt forget, if you are crossing Canada west of Winnipeg in May, look out of the window. ThereÕs probably a good chance that you will see these relict ice-scour features left as 10,000 year furrows on the prairie surface!

References: Morris, T.F. and Kelly, R.I. 1997. Origin and physical and chemical characteristics of glacial overburden in Essex and Kent counties, southwestern Ontario. Canadian Journal of Earth Sciences 34: 233-246.

Teller, J.T. and Clayton, L. (Eds.) 1983. Glacial lake Agassiz. Geological Association of Canada, Special Paper 26.