AT THE HEARTH OF A DEBATE:
THE OUTBURST OF THE ICE AGE
More than 10,000 years ago, ice caps covered northern Europe, Switzerland; and in America, all of Canada and the north of the United States. These ice sheets bestowed these immense glacial landscapes throughout North America and Northern Europe: moraines, drumlins, eskers and s-forms on the bedrock.
For more than a century, geologists and glaciologists believed that these vast glaciers, called ice sheets, disappeared gradually and calmly. But from the '60s, scientists began to suspect that glacial meltwater often swept into gigantic breakouts transforming the topography profoundly under the ice sheet or at its margin. In the following decades, geologists will continue to reveal the scars of these disasters on territories in Scandinavia, the British Isles, Switzerland, Siberia, the Altai Mountains, the Himalayas and Alaska. Even the erosion of the English Channel joins this long list of cataclysms recently.
An illustration of the huge waterfalls cascading over the land bridge that was connecting Britain to France before this ice age catastrophe. Image by Chase Stone / Imperial College London.
This interpretation of these regional reliefs is now a consensus. But in the 1980s, Canadian geologists launched a new controversy. According to them, the glacier landscapes bear the marks of more gigantic catastrophes continental-wide.
Outraged by this idea, some prefer to ignore the debate. But now research in hydrodynamics is constantly reviving this controversy.
Know that ice sheets have shaped our land into two types of terrain:
Those covered by glacial sediments such as moraine.
And bare rocky basements that were carved by the glacier at the time of its disappearance. Often this bedrock is lined with a multitude of the so call s-forms for 'sculpted forms.'
Aerial view of s-forms sculpted bedrock in the area of French River, Ontario. This aerial photo by John Shaw covers nearly 100 meters width of the ground.
These two categories of topography are not without controversy especially when it comes to interpret the huge fields of drumlin. These elongate hills cover 80% of the Canadian Shield area ¾ of Canada. To appreciate the size of the drumlins fields just do a simple tour of Google Earth over the center of the country:
Drumlin fields are also found in the northern United States, the United Kingdom, Poland, Scandinavia, and Switzerland and southern Germany.
In the early 1980s, geologist John Shaw developed a catastrophic subglacial water model to explain drumlin fields and numerous s-forms. Given the vast territories of drumlins and volume estimation of the cataclysmic water at stake, the theory aroused criticism from some glaciologists.
At the same time, Shaw and his colleagues demonstrate that s-forms are sculpted by turbulent flows of water and not by plastic deformation of the glacier base. Since the 1980s, publications and laboratory works have reaffirmed this theory. Today hydrodynamic experts are open to this explanation of these s-forms.
However, some glaciologists still believe that the plastic deformation of the glacier is at the origin of the s-forms. Proponents of this position have not yet produced any laboratory experiment, mathematical model or even analogy with our modern glaciers. Why does this lack of openness persist although the support of hydrodynamicists? The logic is simple:
By accepting that the s-forms have a hydro-catastrophic origin one is forced to glimpse that the origin of drumlins with the same standpoint as both are closely associated. And as the extent of these territories are vast, hydro-catastrophes must be continental. As we will see in the section on drumlins, several field data support the thesis of catastrophic subglacial waters.
With this in mind, three Canadian geoscientists published A flowline map of glaciated Canada, based on new remote sensing data. Two of these researchers, John Shaw and David Sharpe, contributed to the videos and the Geodoxa website.
Between the vast expanses of drumlin fields, large areas of exposed bedrock are mostly covered by s-forms.
We propose an article in which the antagonists are outraged, followed by a reply of the protagonists. The reader will note that the antagonists avoid addressing the subject of the bedrock erosion and that their argument is built largely on philosophy, especially an anti-catastrophe one.
As we will see, the study of bedrock is essential to solve this controversy. When a rocky outcrop is preserved from weathering, cavitation marks associated with the s-forms can be observed. Cavitation indicates that the water reached a violent flow exceeding 10 m / s (see studies by Allen, 1982). Remember that even hard granites of the Canadian Shield are weathered down by atmospheric alteration and the acid of the lichens. So, to better study s-forms we must dwell with sites where the bedrock has been preserved unaltered for 10 000 years just as it was after the disappearance of ice sheet. That's why we'll be looking at sites like French River in Ontario and Cantley in Quebec. Much of French River's rock has been preserved under the water of Georgian Bay since the Ice Age. 800 years ago, the level of the bay suddenly dropped, exposing many pristine s-forms. Cantley is an old sandpit whose exploitation in the 80s exhumed well preserved s-forms.
The study of the traces left by contemporary disasters (sudden flood, jökulhlaup and tsunami) largely support the idea that the glacial s-forms were carved by turbulent waters. In addition, when an alpine glacier gradually recedes, it will reveal a base of a bedrock without s-forms. We will see other marks such as glacial striae. The s-forms will only widen during violent outburst of subglacial waters.
Geodoxa is currently developing a collaboration with various hydrodynamic research institutes around the world. The French River s-forms draw the attention of those colleagues who are working to better understand vortex and cavitation. New knowledge on the subject contributes greatly to the development of new technologies.
The knowledge of s-forms helps also to identify the coastlines that were affected by ancient tsunamis.
The new generation of geoscientists is entitled to wonder why some glaciologists stick to an outdated understanding of s-forms.
We offer a detailed study of s-forms in a format accessible to all.
And, a detailed page on drumlins and moraines.
Allen, J. R. L., 1982, Sedimentary structures: their character and physical basis, Volume 2, New York, Elsevier Scientific Publishing Co., p. 253-291.
Baker V. R. and Pickup Q., 1987, Flood geomorphology of the Katherine
Gorge, Northern Territory, Australia: Geological Society or America
Bulletin, v. 98, p. 635-646.
Bryant E., 2014, Signatures of Tsunami in the Coastal Landscape. In: Tsunami, pp 35-61, Springer, Cham
Kor, P. S. G., Shaw, J. and Sharpe, D. R. (1991). Erosion of bedrock by subglacial meltwater, Georgian Bay, Ontario: a regional view. Canadian Journal of Earth Sciences, 28, 623–642.
Richardson, K., and Carling, P.A., 2005. A Typology of Sculpted Forms in Open Bedrock Channels. Special Paper 392. Geological Society of America, Boulder Colorado, 108 pp.
Sharpe, D.R. and Shaw, J. (1989). Erosion of bedrock by subglacial meltwater, Cantley, Quebec. Geological Society of America Bulletin, 101, 1011–1020.